Basic and Clinical Sciences Examination (BCSE) — Questions and Answers

Question 1: In Wilson's disease, which mineral accumulates pathologically in the liver, brain, and cornea due to impaired biliary excretion?

Iron
Copper (Correct answer)
Zinc
Manganese

Correct answer: Copper

Wilson's disease results from mutations in ATP7B, impairing hepatic copper export into bile and causing toxic copper accumulation in the liver, basal ganglia, and cornea (Kayser-Fleischer rings).

Question 2: A patient with chronic kidney disease (eGFR 15 mL/min) requires pain management. She is already on lisinopril and spironolactone. Which analgesic choice presents the greatest risk for an acute, life-threatening electrolyte disturbance?

Ketorolac 15 mg IV every 6 hours (Correct answer)
Acetaminophen 500 mg every 6 hours
Tramadol 50 mg every 8 hours
Low-dose buprenorphine patch (5 mcg/hr)

Correct answer: Ketorolac 15 mg IV every 6 hours

Ketorolac (an NSAID) inhibits prostaglandin synthesis, which is critical for maintaining renal afferent arteriolar dilation in a patient with CKD. This further reduces GFR in an already compromised kidney. More critically, reduced renal perfusion and prostaglandin inhibition decrease aldosterone effect and potassium excretion. Combined with lisinopril (ACE inhibitor, which reduces aldosterone) and spironolactone (aldosterone antagonist), the patient faces a 'triple whammy' nephrotoxic combination with severe risk of life-threatening hyperkalemia and acute kidney injury. NSAIDs are contraindicated in this scenario.

Question 3: In ruminants, which forestomach compartment is responsible for receiving ingesta directly from the esophagus and initiating fermentation?

Rumen (Correct answer)
Reticulum
Abomasum
Omasum

Correct answer: Rumen

The rumen is the largest forestomach compartment and is the primary site of microbial fermentation of plant material in ruminants.

Question 4: A meta-analysis of 15 randomized trials examining beta-blocker use post-MI produces a funnel plot that is markedly asymmetric, with small studies clustered on one side showing larger effect sizes. The Egger's test is statistically significant (p = 0.03). Which of the following represents the MOST methodologically rigorous next step to address this finding?

Conduct a subgroup analysis stratified by study quality score to determine whether heterogeneity explains the asymmetry
Accept the pooled result as valid because 15 trials provide sufficient statistical power to overcome publication bias
Exclude all small studies (n < 100) from the meta-analysis to eliminate the source of asymmetry
Apply the trim-and-fill method to impute missing studies and recalculate the pooled effect size, then assess whether the adjusted estimate remains clinically significant (Correct answer)

Correct answer: Apply the trim-and-fill method to impute missing studies and recalculate the pooled effect size, then assess whether the adjusted estimate remains clinically significant

Funnel plot asymmetry with a significant Egger's test strongly suggests publication bias (though heterogeneity and true small-study effects are also possible causes). The trim-and-fill method is the standard next step: it imputes the theoretically missing studies on the opposite side of the funnel and recalculates the pooled estimate. If the adjusted estimate remains clinically meaningful, the conclusion is more robust. Simply excluding small studies introduces selection bias; accepting the result ignores potential overestimation; subgroup analysis by quality addresses heterogeneity but not publication bias directly.

Question 5: A 10-year-old mare is admitted for acute, profuse, watery diarrhea, depression, and fever. A complete blood count reveals a packed cell volume (PCV) of 60% and profound neutropenia with a degenerative left shift. What is the most likely cause of the neutropenia?

Immune-mediated destruction of neutrophils in the spleen
Bone marrow suppression due to a chronic viral infection
Hemoconcentration artifactually lowering the neutrophil count
Sequestration of neutrophils into the inflamed colon and margination due to endotoxemia (Correct answer)

Correct answer: Sequestration of neutrophils into the inflamed colon and margination due to endotoxemia

In acute colitis (e.g., Salmonellosis), endotoxemia is a major feature. Endotoxins (LPS) from gram-negative bacteria cause widespread endothelial activation. This leads to neutrophils moving from the circulating pool to the marginated pool along blood vessel walls and migrating into the severely inflamed intestinal tissue. This rapid shift out of circulation results in a profound, acute neutropenia on the CBC.

Question 6: In ruminants, the omasum performs which of the following functions that distinguishes it most from the reticulum?

Absorption of water and electrolytes while grinding ingesta between laminae (Correct answer)
Mechanical mixing and sorting of ingesta by reticular grooves
Fermentation of volatile fatty acids via microbial action
Secretion of intrinsic factor necessary for vitamin B12 absorption

Correct answer: Absorption of water and electrolytes while grinding ingesta between laminae

The omasum (third stomach compartment) is lined with muscular laminae (leaves) that grind and compress ingesta, and its epithelium actively absorbs water, sodium, and potassium. The reticulum is responsible for mechanical sorting and the reticular groove reflex. Fermentation occurs primarily in the rumen. Intrinsic factor in ruminants is secreted by the abomasum.

Question 7: A 24-year-old woman presents with episodic hypertension, headache, and diaphoresis. Urine metanephrines are markedly elevated. Adrenal resection is performed. Histology shows the 'Zellballen' pattern with sustentacular cells at the periphery of cell nests, confirmed by S100 positivity. The tumor is 5.5 cm with no capsular invasion, but Ki-67 is 4% and there is focal vascular invasion. According to the PASS (Pheochromocytoma of the Adrenal Gland Scaled Score) system, which histologic feature most strongly predicts malignant behavior?

Ki-67 proliferation index of 4%
Tumor size greater than 5 cm
Vascular invasion (Correct answer)
Absence of sustentacular cells on S100 staining

Correct answer: Vascular invasion

In the PASS scoring system, vascular invasion carries a score of 1 point and is one of the most clinically significant individual features because it represents direct evidence of aggressive tumor behavior. While large tumor size (>5 cm) contributes to the score, it is less specific for malignancy. A Ki-67 of 4% is borderline but not as weighted in PASS as invasion. The absence of sustentacular cells (loss of S100+ cells) does contribute to PASS, but vascular invasion is the most definitive indicator of malignant potential in pheochromocytoma since benign appearance cannot be guaranteed by histology alone—only metastasis confirms malignancy, making vascular invasion the strongest surrogate marker.

Question 8: Which of the following is the correct sequence for the proper surgical scrub of the patient's skin?

Apply iodine solution without mechanical scrubbing
Scrub in a circular motion from the incision site outward, discarding each sponge after reaching the periphery (Correct answer)
Use a single sponge for the entire scrub area
Scrub from the periphery toward the incision site

Correct answer: Scrub in a circular motion from the incision site outward, discarding each sponge after reaching the periphery

The aseptic scrub technique requires starting at the incision site and moving outward in concentric circles, discarding the sponge once the periphery is reached to avoid recontaminating the center.

Question 9: Dermatophytosis in cats is most commonly caused by which organism?

Microsporum canis (Correct answer)
Microsporum gypseum
Candida albicans
Trichophyton mentagrophytes

Correct answer: Microsporum canis

Microsporum canis accounts for approximately 90% of feline dermatophytosis cases and fluoresces apple-green under a Wood's lamp.

Question 10: A 6-year-old quarter horse presents with acute onset of moderate colic. On rectal palpation, you feel a tightly distended loop of small intestine in the right dorsal quadrant. Which diagnosis is most consistent with these findings?

Right dorsal displacement of the large colon
Duodenal ulceration
Gastric dilation
Ileal impaction (Correct answer)

Correct answer: Ileal impaction

Ileal impaction presents with moderate to severe colic and a palpable, firm, distended loop of small intestine on rectal examination, typically in the right dorsal abdomen.

Question 11: A 4-year-old Thoroughbred gelding is presented for a 2-week history of progressive, asymmetric ataxia and gluteal muscle atrophy. The horse is bright and alert with a normal appetite. Neurologic examination reveals proprioceptive deficits and weakness that are worse in the right hindlimb. What is the most likely causative agent of these clinical signs?

Sarcocystis neurona (Correct answer)
Equine herpesvirus-1
Clostridium tetani
Streptococcus equi

Correct answer: Sarcocystis neurona

The classic clinical presentation for Equine Protozoal Myeloencephalitis (EPM) is progressive, asymmetric neurological deficits, including ataxia and muscle atrophy. Sarcocystis neurona is the most common causative agent of EPM in North America. The other options cause different diseases: Equine herpesvirus-1 typically causes symmetric hindlimb ataxia, Clostridium tetani causes spastic paralysis (tetanus), and Streptococcus equi causes strangles.

Question 12: What is the primary function of the soda lime granules within the canister of a rebreathing anesthetic circuit?

To humidify the inspired gases to prevent desiccation of the airways.
To chemically absorb exhaled carbon dioxide. (Correct answer)
To absorb waste anesthetic gas, preventing environmental pollution.
To vaporize the liquid anesthetic into a gas for delivery to the patient.

Correct answer: To chemically absorb exhaled carbon dioxide.

Soda lime is a chemical absorbent, primarily composed of calcium hydroxide, whose function is to remove carbon dioxide from the exhaled gases in a rebreathing circuit. This allows the other gases (oxygen and anesthetic agent) to be safely rebreathed by the patient, conserving heat, humidity, and anesthetic agent. Waste gas is removed by the scavenger system, and the vaporizer is responsible for turning liquid anesthetic into gas.

Question 13: Which finding on a transtracheal wash or bronchoalveolar lavage is most consistent with eosinophilic bronchopneumopathy in a dog?

Predominance of lymphocytes
Charcot-Leyden crystals with mast cell degranulation
Neutrophilic inflammation with intracellular bacteria
Greater than 25% eosinophils on cytology (Correct answer)

Correct answer: Greater than 25% eosinophils on cytology

Eosinophilic bronchopneumopathy is characterized by eosinophilic airway inflammation, typically >25% eosinophils on BAL cytology.

Question 14: Which toxic effect is associated with methanol poisoning but not ethanol poisoning?

Wernicke encephalopathy due to thiamine depletion
Severe metabolic acidosis with visual disturbances and blindness (Correct answer)
Hypoglycemia due to NAD+ depletion
Central nervous system depression and respiratory failure

Correct answer: Severe metabolic acidosis with visual disturbances and blindness

Methanol is metabolized to formaldehyde and formic acid by alcohol dehydrogenase; formate accumulates in the retina and optic nerve, causing irreversible blindness and severe anion-gap metabolic acidosis.

Question 15: Which nerve is at greatest risk of injury during a perineal urethrostomy in a male cat?

Pudendal nerve (Correct answer)
Femoral nerve
Sciatic nerve
Obturator nerve

Correct answer: Pudendal nerve

The pudendal nerve supplies the perineal region and urethra and is in close proximity to the surgical field during perineal urethrostomy.

Question 16: A 4-year-old Holstein dairy cow presents with a 3-day history of anorexia, decreased milk production, and progressive weakness. Blood work reveals: PCV 12%, total protein 5.2 g/dL, marked macrocytosis, hypersegmented neutrophils, and low serum cobalamin. Which condition BEST explains this presentation?

Bovine leukemia virus-associated lymphoma
Iron deficiency from chronic subclinical hemorrhage
Cobalt deficiency leading to vitamin B12-responsive anemia (Correct answer)
Copper deficiency causing secondary anemia

Correct answer: Cobalt deficiency leading to vitamin B12-responsive anemia

Ruminants cannot synthesize vitamin B12 (cobalamin) directly; they depend on ruminal microbes synthesizing it from dietary cobalt. Cobalt deficiency leads to B12 deficiency, which impairs methylmalonyl-CoA mutase and methionine synthase, producing a megaloblastic (macrocytic) anemia with hypersegmented neutrophils — identical to B12 deficiency in other species. The low serum cobalamin confirms the diagnosis. Copper deficiency causes a normocytic or microcytic anemia without hypersegmented neutrophils. BLV lymphoma does not typically cause megaloblastic changes. Iron deficiency produces microcytic, hypochromic anemia.

Question 17: What is the primary cause of freemartinism in cattle?

Vascular anastomosis between twins of opposite sex sharing testosterone (Correct answer)
Nutritional deficiency during gestation
Chromosomal trisomy in the female twin
Infection with Brucella abortus in utero

Correct answer: Vascular anastomosis between twins of opposite sex sharing testosterone

Freemartinism results from placental vascular anastomoses between male and female twins, allowing testosterone and anti-Müllerian hormone from the male to masculinize the female co-twin.

Question 18: Which fatty acid is classified as omega-3 and is an essential dietary nutrient that cannot be synthesized de novo by mammals?

Arachidonic acid
Alpha-linolenic acid (ALA) (Correct answer)
Oleic acid
Palmitoleic acid

Correct answer: Alpha-linolenic acid (ALA)

Alpha-linolenic acid (18:3n-3) is an essential omega-3 fatty acid because mammals lack the delta-15 desaturase needed to introduce the n-3 double bond.

Question 19: Which enzyme is primarily responsible for metabolizing warfarin in the liver?

CYP3A4
CYP2D6
CYP2C9 (Correct answer)
CYP1A2

Correct answer: CYP2C9

CYP2C9 is the primary hepatic enzyme responsible for the metabolism of the S-enantiomer of warfarin, which accounts for most of its anticoagulant activity.

Question 20: Aminoglycosides exert their antibacterial effect primarily by:

Inhibiting the 50S ribosomal subunit and blocking peptide elongation
Inhibiting cell wall synthesis by binding penicillin-binding proteins
Binding the 30S ribosomal subunit causing misreading of mRNA (Correct answer)
Disrupting the bacterial cell membrane integrity

Correct answer: Binding the 30S ribosomal subunit causing misreading of mRNA

Aminoglycosides irreversibly bind the 30S ribosomal subunit, causing misreading of the genetic code and insertion of incorrect amino acids, ultimately leading to bacterial cell death.

Question 21: Which mechanism allows methicillin-resistant Staphylococcus aureus (MRSA) to resist beta-lactam antibiotics?

Production of an altered penicillin-binding protein (PBP2a) encoded by the mecA gene (Correct answer)
Reduced outer membrane permeability
Production of beta-lactamase that hydrolyzes all beta-lactams
Efflux pump expelling all beta-lactams

Correct answer: Production of an altered penicillin-binding protein (PBP2a) encoded by the mecA gene

MRSA carries the mecA gene encoding PBP2a, a transpeptidase with very low affinity for all beta-lactam antibiotics, conferring broad resistance.

Question 22: The DALY (Disability-Adjusted Life Year) combines:

Disease incidence and prevalence data
Mortality rates and healthcare costs
Years of life lost due to premature mortality and years lived with disability (Correct answer)
Quality of life scores and life expectancy at birth

Correct answer: Years of life lost due to premature mortality and years lived with disability

One DALY = one lost year of healthy life, combining years of life lost (YLL) from premature death and years lived with disability (YLD).

Question 23: Which of the following dietary modifications is most effective in preventing obstructive urolithiasis in male small ruminants, particularly those on high-concentrate diets?

Supplementing with high levels of magnesium
Adding sodium bicarbonate to the feed
Increasing the dietary calcium-to-phosphorus ratio to at least 2:1 (Correct answer)
Feeding a diet high in protein to increase water intake

Correct answer: Increasing the dietary calcium-to-phosphorus ratio to at least 2:1

High-concentrate (grain) diets are often high in phosphorus and low in calcium, which predisposes to the formation of phosphatic (struvite) calculi. Ensuring the dietary calcium-to-phosphorus ratio is at least 2:1 helps prevent the formation of these stones by reducing the amount of phosphorus absorbed and excreted in the urine.

Question 24: A horse presents with mucopurulent nasal discharge and submandibular lymphadenopathy. Culture yields a beta-hemolytic Streptococcus with a Lancefield group C antigen. What is the most likely diagnosis?

Strangles caused by Streptococcus equi subsp. equi (Correct answer)
Strangles caused by Streptococcus equi subsp. zooepidemicus
Pneumonia caused by Streptococcus pneumoniae
Pigeon fever caused by Corynebacterium pseudotuberculosis

Correct answer: Strangles caused by Streptococcus equi subsp. equi

Streptococcus equi subsp. equi (group C Streptococcus) is the obligate pathogen causing classic strangles with abscessation of submandibular lymph nodes in horses.

Question 25: A 6-month-old heifer born co-twin to a bull presents with a shallow vaginal vault (4 cm depth) and no history of observed estrous cycles. Physical examination is otherwise unremarkable. Which diagnostic test provides the MOST definitive confirmation of freemartinism?

Serial progesterone assays 14 days apart showing persistently low values (<1 ng/mL)
Serum anti-Müllerian hormone (AMH) measurement demonstrating elevation above the normal heifer reference range
Karyotyping of peripheral blood lymphocytes demonstrating XX/XY chimerism (Correct answer)
Rectal palpation confirming bilateral uterine horn hypoplasia and absent palpable ovarian structures

Correct answer: Karyotyping of peripheral blood lymphocytes demonstrating XX/XY chimerism

Karyotyping revealing XX/XY chimerism in peripheral blood lymphocytes is the definitive test for freemartinism. Placental vascular anastomoses between heterosexual twins allow bidirectional exchange of hematopoietic progenitor cells, creating persistent blood chimerism detectable throughout the animal's life. AMH may be elevated due to residual gonadal tissue but is not pathognomonic. Low progesterone and clinical findings (shallow vagina, uterine hypoplasia) are consistent but not specific — other causes of sexual infantilism exist and would require different management.

Question 26: A ewe presents in late gestation with twin fetuses. Transrectal palpation reveals one fetus in anterior longitudinal presentation with dorsoilial position (spine oriented toward the ilium rather than the sacrum). What is the correct obstetrical classification of this positional deviation and the MOST appropriate initial correction maneuver?

Dorsosacral position with poll deviation; apply traction to the head while an assistant elevates the dam's hindquarters
Dorsopubic position; repel the fetus, apply a head snare, and rotate 180° using the snare as a lever point
Lateral position; rotate the fetus 90° around its longitudinal axis toward the upper birth canal with counter-pressure on the contralateral hip (Correct answer)
Ventral position; repel the fetus cranially into the uterus and rotate 180° around the longitudinal axis before applying traction

Correct answer: Lateral position; rotate the fetus 90° around its longitudinal axis toward the upper birth canal with counter-pressure on the contralateral hip

In obstetrical terminology, 'position' describes the relationship of the fetal dorsum to the quadrants of the maternal pelvis. Dorsoilial position means the fetal spine faces the maternal ilium (lateral), placing this as a lateral position — not dorsosacral (normal) or dorsopubic (ventral/upside-down). The correction for lateral position is rotation of the fetus approximately 90° around its own longitudinal axis to bring the dorsum to the dorsosacral (normal) position. This is accomplished by pushing on the fetal hip or shoulder while rotating. Repelling 180° would be appropriate for dorsopubic (ventral) position, not lateral.

Question 27: A 55-year-old man with hepatitis C cirrhosis develops ascites and jaundice. AFP is markedly elevated. Imaging reveals a 4-cm arterially enhancing liver mass. What is the most likely diagnosis?

Hepatic hemangioma
Metastatic colon cancer
Hepatocellular carcinoma (Correct answer)
Cholangiocarcinoma

Correct answer: Hepatocellular carcinoma

Hepatocellular carcinoma in the setting of cirrhosis characteristically shows arterial enhancement on imaging and markedly elevated alpha-fetoprotein (AFP).

Question 28: A randomized controlled trial of a vaccine shows an efficacy of 80%. In a subsequent outbreak, the attack rate among unvaccinated individuals is 25%, while the attack rate among vaccinated individuals is 10%. However, 70% of the affected community is vaccinated. What is the vaccine effectiveness under these field conditions, and which phenomenon best explains the discrepancy from the trial efficacy?

Field VE = 60%; explained by healthy vaccinee bias and cold-chain failure reducing real-world protection (Correct answer)
Field VE = 75%; explained by herd immunity reducing exposure in the unvaccinated group
Field VE = 80%; the field and trial results are concordant, with no discrepancy present
Field VE = 60%; primarily explained by waning immunity over time since trial completion

Correct answer: Field VE = 60%; explained by healthy vaccinee bias and cold-chain failure reducing real-world protection

Field VE = 1 − (Attack Rate Vaccinated / Attack Rate Unvaccinated) = 1 − (10/25) = 1 − 0.4 = 60%. This is lower than the trial efficacy of 80%. In field conditions, healthy vaccinee bias (healthier individuals more likely to be vaccinated) can paradoxically inflate observed benefit in trials, while cold-chain failures, immunosenescence, and variant drift reduce real-world protection. The most operationally addressable explanation in outbreak investigations is cold-chain compromise alongside healthy vaccinee bias.

Question 29: A lymph node biopsy from a 34-year-old HIV-positive man shows paracortical expansion by large cells with prominent nucleoli, abundant pale cytoplasm, and interspersed small lymphocytes. EBER in-situ hybridization is positive. CD30 is positive; CD15 is negative; CD20 is dim positive; PAX5 is weakly positive; CD45 is positive. What is the most likely diagnosis?

EBV-positive diffuse large B-cell lymphoma, NOS (Correct answer)
Classic Hodgkin lymphoma, mixed cellularity subtype
EBV-positive polymorphic B-cell lymphoproliferative disorder
Plasmablastic lymphoma

Correct answer: EBV-positive diffuse large B-cell lymphoma, NOS

EBV-positive DLBCL, NOS (formerly EBV-positive DLBCL of the elderly, now recognized across age groups especially in immunocompromised patients) characteristically shows CD20 dim positivity, PAX5 weak positivity, CD45 positivity, and CD30 positivity with EBER+. CD15 negativity and retained B-cell markers distinguish it from classic Hodgkin lymphoma. Plasmablastic lymphoma typically loses CD20/PAX5 entirely and is EBER+ but shows plasmacytic differentiation markers.

Question 30: A researcher studies the blood supply to the femoral head in adults. Which artery contributes the LEAST to femoral head perfusion after skeletal maturity, making its disruption less clinically significant?

Inferior retinacular artery
Superior retinacular artery
Artery of the ligamentum teres (obturator artery branch) (Correct answer)
Ascending branch of the lateral circumflex femoral artery

Correct answer: Artery of the ligamentum teres (obturator artery branch)

In adults, the artery of the ligamentum teres (a branch of the obturator artery) contributes minimally to femoral head perfusion — it is functionally important in children but becomes negligible after skeletal maturity and physeal closure. The dominant blood supply to the adult femoral head comes from the medial circumflex femoral artery via the retinacular arteries (superior and inferior), particularly the superior retinacular vessels. This is why intracapsular femoral neck fractures in adults carry high risk of avascular necrosis — they disrupt the retinacular supply while the ligamentum teres contribution is insufficient to compensate.

Question 31: Which organism causes 'Potomac horse fever' (equine monocytic ehrlichiosis) in the United States?

Neorickettsia risticii (Correct answer)
Ehrlichia ruminantium
Rickettsia rickettsii
Anaplasma phagocytophilum

Correct answer: Neorickettsia risticii

Neorickettsia risticii (formerly Ehrlichia risticii) is the causative agent of Potomac horse fever, transmitted via ingestion of infected aquatic insects.

Question 32: In cattle, prolonged retention of the fetal membranes is defined as failure to expel the placenta within how many hours after parturition?

24 hours (Correct answer)
6 hours
48 hours
12 hours

Correct answer: 24 hours

Retained fetal membranes in cattle are defined as placental retention beyond 24 hours post-calving.

Question 33: Which muscle forms the floor of the anatomical snuffbox?

Abductor pollicis longus
Extensor pollicis brevis
Extensor pollicis longus
Scaphoid bone (Correct answer)

Correct answer: Scaphoid bone

The scaphoid bone (and trapezium) forms the floor of the anatomical snuffbox, and tenderness there after a fall suggests a scaphoid fracture.

Question 34: A veterinarian needs to provide standing sedation for a 650 kg dairy cow. When choosing to use xylazine, an alpha-2 adrenergic agonist, which of the following statements is most accurate?

The primary, FDA-approved reversal agent for xylazine in cattle is atipamezole.
Xylazine provides potent sedation in cattle but has no analgesic properties.
Cattle are highly sensitive to xylazine, requiring approximately one-tenth the dose used in horses. (Correct answer)
Cattle are highly resistant to xylazine, requiring doses 10 times higher than those for horses.

Correct answer: Cattle are highly sensitive to xylazine, requiring approximately one-tenth the dose used in horses.

Ruminants, especially cattle, are exceptionally sensitive to the sedative and analgesic effects of xylazine. The dose required for sedation is approximately 1/10th of the dose needed for a horse. This species-specific sensitivity is a critical safety consideration. Tolazoline is the more commonly used and approved reversal agent in cattle, though others may be used off-label.

Question 35: A 6-year-old intact male cat undergoes perineal urethrostomy. Two weeks postoperatively the owner reports urine scalding and a narrow urinary stream. Retrograde urethrogram reveals a focal stricture at the anastomotic site. Which suture technique error during the original procedure is MOST likely responsible?

Failure to spatulate the urethral opening prior to anastomosis
Excessive tension on interrupted sutures placed too close to the urethral mucosa edge, causing ischemic necrosis (Correct answer)
Use of rapidly absorbable suture material (poliglecaprone 25) rather than polyglycolic acid
Placement of too few sutures, allowing mucosal eversion and urothelial exposure

Correct answer: Excessive tension on interrupted sutures placed too close to the urethral mucosa edge, causing ischemic necrosis

Stricture formation at a perineal urethrostomy is classically caused by ischemic necrosis from sutures placed under excessive tension or too close to the delicate urethral mucosal edge, interrupting the marginal blood supply. This leads to fibrosis and luminal narrowing. While spatulation is important in ureteral anastomoses, PU technique relies on precise mucosal apposition. Poliglecaprone 25 is actually appropriate for this procedure; the issue is technique, not material choice.

Question 36: In occupational health, the Threshold Limit Value (TLV) represents:

The minimum detectable concentration of a substance
The maximum airborne concentration to which nearly all workers can be repeatedly exposed without adverse effects (Correct answer)
The concentration requiring immediate evacuation
The lethal concentration for 50% of exposed animals

Correct answer: The maximum airborne concentration to which nearly all workers can be repeatedly exposed without adverse effects

TLVs are guidelines set by ACGIH representing levels for repeated daily occupational exposure without adverse health effects for nearly all workers.

Question 37: A cohort study follows 4,000 non-smokers and 2,000 smokers for 10 years. At the end, 40 non-smokers and 60 smokers develop lung cancer. What is the population attributable risk percent (PAR%) for smoking, assuming a smoking prevalence of 30% in the general population?

28.6%
50.0%
60.0%
37.5% (Correct answer)

Correct answer: 37.5%

Incidence in smokers = 60/2000 = 3%. Incidence in non-smokers = 40/4000 = 1%. RR = 3. PAR% = [Pe(RR-1)] / [Pe(RR-1) + 1] × 100, where Pe = 0.30. Numerator = 0.30 × (3-1) = 0.60. Denominator = 0.60 + 1 = 1.60. PAR% = 0.60/1.60 × 100 = 37.5%. This reflects the proportion of lung cancer in the total population attributable to smoking.

Question 38: During a cystotomy closure, which suture pattern is recommended for the bladder wall to provide a watertight seal?

Simple continuous in a single layer through all layers
Two-layer closure with an inverting pattern such as Cushing or Lembert over a simple continuous (Correct answer)
Stapled closure only
Mattress sutures through all layers

Correct answer: Two-layer closure with an inverting pattern such as Cushing or Lembert over a simple continuous

A two-layer closure using a simple continuous first layer followed by an inverting Cushing or Lembert pattern provides a secure, watertight, leak-resistant bladder closure.

Question 39: A patient with chronic kidney disease (GFR 18 mL/min) requires analgesia. They are prescribed tramadol. Which pharmacokinetic and pharmacodynamic concern makes this combination particularly dangerous?

Reduced renal prostaglandin synthesis in CKD enhances tramadol's COX-inhibiting effects, causing acute tubular necrosis
Tramadol undergoes extensive renal tubular secretion, leading to competitive inhibition of creatinine excretion and falsely elevated serum creatinine
Active O-desmethyltramadol (M1 metabolite, a μ-opioid agonist) and the parent compound accumulate due to impaired renal clearance, increasing seizure and opioid toxicity risk (Correct answer)
CKD causes hypoalbuminemia that increases free tramadol levels, potentiating serotonin syndrome

Correct answer: Active O-desmethyltramadol (M1 metabolite, a μ-opioid agonist) and the parent compound accumulate due to impaired renal clearance, increasing seizure and opioid toxicity risk

Tramadol is partially converted by CYP2D6 to O-desmethyltramadol (M1), which has ~200-fold higher μ-opioid receptor affinity than the parent drug. Both tramadol and M1 are renally excreted. In CKD (especially GFR <30), their half-lives are prolonged dramatically, leading to accumulation. This raises risk of two distinct toxicities: (1) opioid toxicity from M1 accumulation (respiratory depression, CNS depression), and (2) seizures, since tramadol itself lowers seizure threshold via GABA modulation and norepinephrine/serotonin reuptake inhibition. Tramadol is generally contraindicated or used with extreme caution at GFR <30.

Question 40: What is the primary cause of freemartinism in cattle?

Hormonal imbalance during mid-gestation
Vascular anastomosis between heterosexual twins sharing a chorion (Correct answer)
Autosomal recessive gene on the X chromosome
Chromosomal trisomy in the female twin

Correct answer: Vascular anastomosis between heterosexual twins sharing a chorion

Freemartinism results from shared placental circulation between male and female twins, allowing testosterone and anti-Müllerian hormone to masculinize the female.

Question 41: In the diagnosis of Johne's disease (paratuberculosis) in cattle, which test has the highest sensitivity for detecting shedding animals in the subclinical phase?

Fecal PCR (Correct answer)
Serum ELISA antibody test
Fecal culture
Intradermal johnin test

Correct answer: Fecal PCR

Fecal PCR has higher sensitivity than fecal culture for detecting subclinical shedders, with results available in days rather than the weeks required for culture.

Question 42: A 2-year-old domestic shorthair cat is presented with a history of acute vomiting, anorexia, and abdominal pain. A physical exam reveals a string-like object under the tongue. Abdominal radiographs show bunching of the small intestines in the mid-abdomen with multiple, abnormally shaped, comma-like or triangular gas bubbles. What is the classic term for this radiographic appearance?

Gravel sign
Plication (Correct answer)
Sentinel loop
Ileus

Correct answer: Plication

The clinical history of a string under the tongue combined with the radiographic findings of bunched intestines and oddly shaped gas pockets is pathognomonic for a linear foreign body. The bunching or gathering of the intestine along the linear object is termed plication. The 'gravel sign' refers to an accumulation of radiopaque material in the intestine, often seen with partial obstructions. A 'sentinel loop' is a focal, gas-distended segment of intestine indicating nearby inflammation. Ileus describes a general lack of intestinal motility, leading to diffuse gas distention, not the specific bunching pattern seen here.

Question 43: The dorsal root ganglion (DRG) contains the cell bodies of which type of neuron?

First-order sensory neurons (Correct answer)
Upper motor neurons
Preganglionic sympathetic neurons
Lower motor neurons

Correct answer: First-order sensory neurons

The DRG houses pseudounipolar first-order sensory neuron cell bodies that transmit afferent signals from peripheral receptors to the spinal cord.

Question 44: A horse with confirmed equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona is started on treatment. Which drug combination is FDA-approved specifically for EPM treatment in horses?

Metronidazole and rifampin
Both B and C are approved options (Correct answer)
Pyrimethamine and sulfadiazine
Ponazuril or diclazuril monotherapy

Correct answer: Both B and C are approved options

Ponazuril (Marquis), diclazuril (Protazil), and the combination of pyrimethamine/sulfadiazine (Re-Balance) are all FDA-approved treatments for EPM in horses.

Question 45: Glanders (Burkholderia mallei) is suspected in an imported horse. Which characteristic most reliably distinguishes B. mallei from its close relative Burkholderia pseudomallei (melioidosis agent) in laboratory identification?

B. mallei is non-motile and arginine dihydrolase-negative, whereas B. pseudomallei is motile with a polar tuft of flagella (Correct answer)
B. mallei ferments lactose rapidly on MacConkey agar, whereas B. pseudomallei is a non-fermenter
B. mallei produces a distinctive violet pigment on MacConkey agar not seen with B. pseudomallei
B. mallei is oxidase-negative whereas B. pseudomallei is oxidase-positive

Correct answer: B. mallei is non-motile and arginine dihydrolase-negative, whereas B. pseudomallei is motile with a polar tuft of flagella

B. mallei and B. pseudomallei are genetically nearly identical (B. mallei is essentially a host-adapted clone of B. pseudomallei), making differentiation challenging. The key phenotypic distinctions are: B. mallei is non-motile (lacks flagellar genes deleted during host adaptation) and is arginine dihydrolase-negative, while B. pseudomallei is motile via a bipolar tuft of flagella and is arginine dihydrolase-positive. Both are oxidase-positive, Gram-negative, and both fail to ferment lactose. Neither produces violet pigment. Both are Tier 1 select agents requiring BSL-3 containment.

Question 46: Which sperm morphological defect is classified as a MAJOR defect and would reduce a bull's BSE score?

Slightly bent midpiece
Distal cytoplasmic droplets
Minor head shape variation
Proximal cytoplasmic droplets (Correct answer)

Correct answer: Proximal cytoplasmic droplets

Proximal cytoplasmic droplets are major defects indicating incomplete sperm maturation, whereas distal droplets are minor defects that may indicate rapid transit through the epididymis.

Question 47: A 4-year-old captive chimpanzee repeatedly rocks back and forth, self-grasps, and displays pica behavior. These stereotypies most likely indicate which underlying welfare concern?

Subclinical viral encephalitis causing basal ganglia dysfunction
Chronic psychological stress due to inadequate environmental enrichment and social deprivation (Correct answer)
Dominance-asserting displacement behavior directed at subordinate group members
Normal developmental behavior associated with adolescent cognitive restructuring

Correct answer: Chronic psychological stress due to inadequate environmental enrichment and social deprivation

Stereotypies in non-human primates—repetitive, invariant behaviors with no apparent function—are hallmark indicators of chronic psychological stress, typically arising from inadequate environmental complexity, social isolation, or early maternal deprivation. Rocking, self-grasping, and pica are classic examples documented extensively in deprived laboratory and zoo primates. Viral encephalitis would present with additional neurological signs, and stereotypies are not considered normal developmental behavior.

Question 48: A 4-year-old Thoroughbred gelding is evaluated for intermittent exercise intolerance and post-exertional muscle stiffness. Serum CK peaks at 48,000 IU/L four hours after moderate exercise. A muscle biopsy reveals abnormal polysaccharide inclusions that stain positively with PAS but are resistant to diastase digestion. Genetic testing is NEGATIVE for the GYS1 mutation. Which condition is most likely responsible?

Polysaccharide storage myopathy type 1 (PSSM1)
Polysaccharide storage myopathy type 2 (PSSM2) (Correct answer)
Recurrent exertional rhabdomyolysis (RER)
Malignant hyperthermia

Correct answer: Polysaccharide storage myopathy type 2 (PSSM2)

PSSM type 1 is caused by a gain-of-function mutation in the GYS1 gene (glycogen synthase) and produces diastase-resistant PAS-positive inclusions. However, since GYS1 testing is negative, PSSM type 2 is the correct diagnosis. PSSM2 has similar histopathological findings (diastase-resistant PAS-positive inclusions) but is not caused by the GYS1 mutation; its genetic basis involves other variants (e.g., MYH1 mutations in some cases). RER typically does not produce abnormal polysaccharide inclusions on biopsy. Malignant hyperthermia is triggered by volatile anesthetics and is not associated with polysaccharide storage on biopsy.

Question 49: Ivermectin exerts its antiparasitic effect primarily by:

Blocking acetylcholinesterase at the neuromuscular junction
Inhibiting tubulin polymerization in helminths
Disrupting mitochondrial electron transport in protozoa
Potentiating glutamate-gated chloride channels, causing paralysis (Correct answer)

Correct answer: Potentiating glutamate-gated chloride channels, causing paralysis

Ivermectin binds to glutamate-gated chloride channels (and to a lesser degree GABA-gated channels) in invertebrate nerve and muscle cells, causing hyperpolarization and irreversible paralysis. Benzimidazoles inhibit tubulin polymerization; organophosphates block acetylcholinesterase; and antiprotozoals like atovaquone target mitochondrial transport.

Question 50: Nesting behavior in sows is most compromised by which housing system?

Deep-litter outdoor systems
Hoop-barn group housing
Gestation crates that prevent movement and turning (Correct answer)
Group housing with straw bedding

Correct answer: Gestation crates that prevent movement and turning

Gestation crates prevent the sow from performing motivated pre-farrowing nesting movements, causing chronic frustration and representing a major animal welfare concern.

Question 51: Which antifungal mechanism of action does amphotericin B use to kill fungal cells?

Blocks fungal DNA gyrase
Inhibits beta-glucan synthase in the cell wall
Inhibits lanosterol 14-alpha-demethylase
Binds ergosterol, forming pores that disrupt membrane integrity (Correct answer)

Correct answer: Binds ergosterol, forming pores that disrupt membrane integrity

Amphotericin B binds ergosterol in the fungal cell membrane and creates transmembrane pores, causing lethal leakage of intracellular ions.

Question 52: A 25-year-old male presents with a mediastinal mass. Biopsy shows large cells with bilobed nuclei and prominent eosinophilic 'owl-eye' nucleoli in a background of lymphocytes, eosinophils, and plasma cells. What is the diagnosis?

T-lymphoblastic lymphoma
Primary mediastinal large B-cell lymphoma
Nodular sclerosis Hodgkin lymphoma (Correct answer)
Diffuse large B-cell lymphoma

Correct answer: Nodular sclerosis Hodgkin lymphoma

Reed-Sternberg cells with owl-eye nucleoli in an inflammatory background define classical Hodgkin lymphoma, with nodular sclerosis being most common in young adults with mediastinal involvement.

Question 53: According to the 3Rs framework, 'refinement' specifically refers to:

Using genetically modified animals for efficiency
Decreasing the total number of animals used
Replacing animals with non-sentient alternatives
Improving procedures to minimize pain, suffering, and distress (Correct answer)

Correct answer: Improving procedures to minimize pain, suffering, and distress

Refinement encompasses any modification of husbandry or experimental procedures that reduces the severity or duration of suffering experienced by animals.

Question 54: Which physiologic mechanism best explains the development of hypocalcemia in dairy cows fed high-cation-anion difference (DCAD) diets in late gestation?

High sodium competitively inhibits calcium reabsorption in the kidney
Metabolic alkalosis reduces tissue sensitivity to PTH and decreases intestinal calcium absorption (Correct answer)
High potassium suppresses PTH secretion directly
Dietary calcium binds to excess potassium forming insoluble complexes

Correct answer: Metabolic alkalosis reduces tissue sensitivity to PTH and decreases intestinal calcium absorption

Dietary-induced metabolic alkalosis decreases PTH receptor sensitivity and reduces 1,25-dihydroxyvitamin D production, impairing intestinal calcium absorption and bone calcium mobilization.

Question 55: A patient develops tachycardia, hyperthermia, muscle rigidity, and rhabdomyolysis after sevoflurane exposure. Temperature is 41°C. What is the immediate treatment?

Cool the patient with ice packs and reassess
Administer beta-blockers for tachycardia control
Administer dantrolene 2.5 mg/kg IV immediately (Correct answer)
Increase volatile anesthetic concentration to deepen anesthesia

Correct answer: Administer dantrolene 2.5 mg/kg IV immediately

Malignant hyperthermia requires immediate dantrolene (2.5 mg/kg IV, repeat up to 10 mg/kg), discontinuation of triggers, and aggressive cooling.

Question 56: In canine autoimmune hemolytic anemia (AIHA), erythrocyte destruction occurs primarily via which mechanism?

Type IV hypersensitivity cytotoxic T-cell attack on erythrocytes
Type II hypersensitivity — IgG/IgM antibodies against RBC surface antigens leading to complement lysis and phagocytosis (Correct answer)
Type III hypersensitivity immune complex deposition in splenic sinusoids
Type I hypersensitivity degranulation of basophils

Correct answer: Type II hypersensitivity — IgG/IgM antibodies against RBC surface antigens leading to complement lysis and phagocytosis

AIHA is a classic Type II hypersensitivity. Auto-antibodies (typically IgG) bind erythrocyte surface antigens; IgM can also fix complement directly. Opsonized RBCs are phagocytosed by splenic and hepatic macrophages (extravascular hemolysis) or lysed by the membrane attack complex (intravascular hemolysis). Spherocytes on blood smear and a positive Coombs test are hallmarks.

Question 57: A 58-year-old man undergoes nephrectomy for a renal mass. Histology reveals tumor cells with abundant clear cytoplasm arranged in nests separated by a delicate vascular network. Immunohistochemistry shows positivity for CD10, vimentin, and carbonic anhydrase IX (CAIX), but negativity for CK7 and TFE3. A deletion in which chromosomal region is most characteristic of this tumor type?

Deletion of chromosome 11p13 (WT1 locus)
Translocation t(X;1)(p11;q21) involving TFE3
Deletion of chromosome 3p (VHL locus) (Correct answer)
Monosomy of chromosome 1

Correct answer: Deletion of chromosome 3p (VHL locus)

The described tumor is clear cell renal cell carcinoma (ccRCC), the most common renal malignancy. The characteristic molecular alteration is loss of heterozygosity at chromosome 3p25, where the VHL (von Hippel-Lindau) tumor suppressor gene resides. VHL inactivation leads to constitutive HIF activation and downstream upregrowth of VEGF and PDGF. CAIX positivity reflects HIF-driven transcription. TFE3 translocation at Xp11 characterizes Xp11 translocation RCC, a separate entity with TFE3 positivity. WT1 deletion is associated with Wilms tumor. Monosomy 1 is seen in chromophobe RCC along with monosomies of chromosomes 2, 6, 10, 13, and 17.

Question 58: In mammals, which region of the nephron is primarily responsible for active sodium reabsorption driven by Na+/K+-ATPase on the basolateral membrane?

Loop of Henle thin descending limb
Proximal convoluted tubule (Correct answer)
Glomerulus
Collecting duct

Correct answer: Proximal convoluted tubule

The proximal convoluted tubule reabsorbs approximately 65–70% of filtered sodium via Na+/K+-ATPase on the basolateral surface, with other solutes following osmotically.

Question 59: A 4-year-old intact male cat presents with urethral obstruction. After relief and stabilization, bloodwork reveals severe hyperkalemia causing bradycardia. Physiologically, which mechanism best explains why hyperkalemia depresses cardiac automaticity and slows conduction in feline myocardium?

Elevated extracellular K⁺ activates the Na⁺/K⁺-ATPase pump maximally, causing excessive Na⁺ extrusion that hyperpolarizes nodal pacemaker cells and suppresses spontaneous depolarization
Elevated extracellular K⁺ hyperpolarizes the resting membrane potential of nodal cells by increasing the electrochemical gradient for K⁺ efflux, making it harder to reach threshold
Elevated extracellular K⁺ prolongs phase 2 of the action potential by competitively blocking L-type Ca²⁺ channels, slowing repolarization and extending the refractory period
Elevated extracellular K⁺ reduces the resting membrane potential (makes it less negative) by decreasing the K⁺ equilibrium potential, which inactivates voltage-gated fast Na⁺ channels and reduces phase 0 upstroke velocity (Correct answer)

Correct answer: Elevated extracellular K⁺ reduces the resting membrane potential (makes it less negative) by decreasing the K⁺ equilibrium potential, which inactivates voltage-gated fast Na⁺ channels and reduces phase 0 upstroke velocity

Extracellular potassium concentration directly determines the K⁺ equilibrium potential (Ek) via the Nernst equation. As extracellular K⁺ rises, Ek becomes less negative, which depolarizes the resting membrane potential (RMP) of all cardiac cells. While this initial, modest depolarization might paradoxically seem excitatory, the sustained partial depolarization causes voltage-gated fast Na⁺ channels (responsible for phase 0 in working myocytes and ventricular conduction) to enter an inactivated (non-conducting) state — they cannot open again without full repolarization. The result is reduced phase 0 upstroke velocity (dV/dt), slower conduction velocity, widened QRS, and bradycardia. Option A is incorrect because hyperkalemia depolarizes, not hyperpolarizes, the RMP. Options C and D describe mechanisms not primarily responsible for hyperkalemia's cardiac effects.

Question 60: Which enzyme deficiency leads to phenylketonuria (PKU), resulting in accumulation of phenylalanine?

Homogentisate oxidase
Fumarylacetoacetase
Tyrosine hydroxylase
Phenylalanine hydroxylase (Correct answer)

Correct answer: Phenylalanine hydroxylase

PKU results from deficiency of phenylalanine hydroxylase, the enzyme that converts phenylalanine to tyrosine in the liver.

Question 61: A 7-year-old Angus cow presents with a 3-week history of progressive weight loss, tenesmus, and intermittent blood-tinged feces. Rectal examination reveals a firm, irregular intraluminal mass approximately 15 cm cranial to the anus. Biopsy histopathology shows nests of cells with squamous differentiation and keratin pearls invading the submucosa. Serum calcium is 14.2 mg/dL (reference: 8.4–10.2 mg/dL). Which mechanism most likely accounts for the hypercalcemia?

Primary hyperparathyroidism concurrent with rectal adenocarcinoma
Tumor secretion of parathyroid hormone-related protein (PTHrP) (Correct answer)
Osteolytic bone metastases releasing calcium into circulation
Ectopic secretion of 1,25-dihydroxyvitamin D by tumor-infiltrating macrophages

Correct answer: Tumor secretion of parathyroid hormone-related protein (PTHrP)

The histopathology describes squamous cell carcinoma (SCC) of the rectum. Humoral hypercalcemia of malignancy (HHM) is the most common cause of paraneoplastic hypercalcemia in large animals, and it is mediated by PTHrP secreted by the tumor. PTHrP mimics PTH at the PTH receptor, causing increased bone resorption, increased renal calcium reabsorption, and decreased renal phosphorus reabsorption. Osteolytic metastases (local osteolysis) are less common in cattle and would be unlikely to cause hypercalcemia of this degree from a rectal primary. Ectopic 1,25-dihydroxyvitamin D production occurs with lymphoma in some species but is not the typical mechanism with SCC. Concurrent primary hyperparathyroidism would be a highly unlikely coincidence.

Question 62: A patient presents with weakness of shoulder abduction beyond 90° and difficulty with overhead activities. Imaging reveals compression at the spinoglenoid notch. Which specific branch of which nerve is most likely affected?

Anterior branch of the axillary nerve
Posterior cord terminal branch of the radial nerve
Inferior branch of the suprascapular nerve
Deep branch of the suprascapular nerve (Correct answer)

Correct answer: Deep branch of the suprascapular nerve

The suprascapular nerve passes through the suprascapular notch to innervate the supraspinatus, then wraps around the spine of the scapula at the spinoglenoid notch to innervate the infraspinatus. Compression at the spinoglenoid notch specifically affects the infraspinatus branch (deep branch), causing isolated infraspinatus weakness. Since the infraspinatus is critical for external rotation and assists in abduction beyond 90°, this results in the described deficit without affecting supraspinatus function.

Question 63: Which ultrasound finding is most consistent with acute cholecystitis rather than chronic cholecystitis?

Positive sonographic Murphy's sign with gallstones and gallbladder wall thickening >3 mm (Correct answer)
Gallstones alone without wall thickening
Gallbladder polyps <5 mm
Echogenic bile sludge without tenderness

Correct answer: Positive sonographic Murphy's sign with gallstones and gallbladder wall thickening >3 mm

Acute cholecystitis is indicated by the combination of sonographic Murphy's sign, gallstones, and wall thickening over 3 mm.

Question 64: A young German Shepherd is presented with a history of chronic diarrhea and significant weight loss despite a ravenous appetite. Which of the following clinical signs is most characteristic of exocrine pancreatic insufficiency (EPI)?

Polyuria and polydipsia
Steatorrhea and polyphagia (Correct answer)
Jaundice and ascites
Melena and intermittent vomiting

Correct answer: Steatorrhea and polyphagia

Exocrine pancreatic insufficiency (EPI) is a condition of maldigestion caused by insufficient production of digestive enzymes by the pancreas. The classic clinical signs include polyphagia (ravenous appetite), significant weight loss, and steatorrhea (large volumes of pale, fatty, malodorous feces) due to the inability to digest fats.

Question 65: Which serologic test is used as the official screening test for brucellosis in US cattle?

Milk ring test only
PCR on whole blood
Complement fixation test only
Rose Bengal plate test (RBPT) or buffered Brucella antigen test (Correct answer)

Correct answer: Rose Bengal plate test (RBPT) or buffered Brucella antigen test

The buffered Brucella antigen (Rose Bengal plate) test is the official US screening test for bovine brucellosis in the national eradication program.

Question 66: A state health department is evaluating whether to implement a mandatory iodine fortification program for table salt to address subclinical iodine deficiency (median urinary iodine 85 µg/L) in a population where 22% of pregnant women have iodine levels below 50 µg/L. An independent assessment finds that the current voluntary fortification program covers only 40% of salt sold. Using the PRECEDE-PROCEED model, which phase is being addressed by quantifying the urinary iodine levels and pregnancy-specific burden, and what is the MOST critical barrier to address next?

Epidemiological assessment phase; the most critical barrier is enabling factors — specifically, the inadequate market penetration of voluntarily fortified salt (Correct answer)
Social assessment phase; the critical barrier is reinforcing factors, specifically inadequate clinician counseling about iodine supplementation
Administrative assessment phase; the critical barrier is policy factors, specifically lack of legislative authority for mandatory fortification
Behavioral assessment phase; the critical barrier is predisposing factors, specifically consumer knowledge deficits about iodine

Correct answer: Epidemiological assessment phase; the most critical barrier is enabling factors — specifically, the inadequate market penetration of voluntarily fortified salt

In the PRECEDE-PROCEED model, quantifying disease burden and at-risk subgroups (median urinary iodine, % of pregnant women deficient) constitutes the Epidemiological Assessment phase (Phase 2). The data show that voluntary fortification reaches only 40% of salt — this is an enabling factor (availability and accessibility of the health-promoting product), not a knowledge deficit or policy gap. Enabling factors are environmental and resource conditions that make healthy behavior possible. The most actionable next step — shifting to mandatory fortification or dramatically expanding distribution — directly targets this enabling barrier. Predisposing (knowledge/attitudes) and reinforcing (social support/clinician counseling) factors are secondary when product availability itself is the bottleneck.

Question 67: A 55-year-old smoker develops hemoptysis. Bronchoscopy reveals a central endobronchial mass. Histology shows small cells with nuclear molding, finely granular 'salt-and-pepper' chromatin, absent nucleoli, and a high mitotic rate with abundant necrosis. IHC is positive for synaptophysin, chromogranin A, and TTF-1, but negative for CK7. Which additional finding would most strongly support a diagnosis of small cell lung carcinoma over atypical carcinoid?

Positive CD56 immunostaining
Ki-67 proliferation index greater than 50% (Correct answer)
Presence of crush artifact on biopsy specimen
Azzopardi effect (DNA encrustation of vessel walls)

Correct answer: Ki-67 proliferation index greater than 50%

The WHO classification distinguishes pulmonary neuroendocrine tumors by mitotic rate and Ki-67: typical carcinoid <2 mitoses/2mm² and Ki-67 <5%; atypical carcinoid 2–10 mitoses/2mm² and Ki-67 up to ~20%; SCLC and large cell NEC have Ki-67 typically >50–80%. A Ki-67 >50% definitively places the tumor in the high-grade category. Crush artifact and Azzopardi effect are characteristic morphologic features of SCLC but are also seen in atypical carcinoid in small biopsies. CD56 is expressed across the neuroendocrine spectrum and does not discriminate grade.

Question 68: Which metabolic pathway is primarily responsible for synthesizing glucose from amino acids during prolonged fasting?

Gluconeogenesis (Correct answer)
Glycolysis
Pentose phosphate pathway
Glycogenolysis

Correct answer: Gluconeogenesis

Gluconeogenesis converts glucogenic amino acids (e.g., alanine, glutamine) into glucose in the liver during prolonged fasting when glycogen stores are depleted.

Question 69: Ultrasound of the liver reveals a 3 cm lesion with posterior acoustic enhancement and uniform anechoic internal echogenicity. Contrast-enhanced CT shows peripheral nodular enhancement in the arterial phase that fills in centripetally on delayed imaging. This enhancement pattern is MOST characteristic of:

Hepatic adenoma with heterogeneous enhancement and intralesional fat
Focal nodular hyperplasia with central scar and spoke-wheel vascularity
Cavernous hemangioma with progressive centripetal fill-in (Correct answer)
Hepatocellular carcinoma with arterial hypervascularity and washout

Correct answer: Cavernous hemangioma with progressive centripetal fill-in

Cavernous hemangiomas classically demonstrate peripheral nodular enhancement in the arterial phase with progressive centripetal fill-in on delayed images, reflecting slow blood pooling in large vascular spaces. On ultrasound, they appear uniformly hyperechoic (in fatty livers) or anechoic with posterior acoustic enhancement. HCC shows arterial enhancement with venous phase washout. FNH has a central scar and spoke-wheel pattern. Adenoma shows heterogeneous enhancement, often with fat or hemorrhage.

Question 70: Parvovirus type 2 in dogs primarily damages which tissue to cause hemorrhagic enteritis?

Colonic goblet cells
Gastric parietal cells
Duodenal Brunner's glands
Intestinal crypt epithelium (Correct answer)

Correct answer: Intestinal crypt epithelium

Canine parvovirus-2 targets rapidly dividing intestinal crypt epithelial cells, destroying the mucosal renewal mechanism and leading to hemorrhagic enteritis.

Question 71: Which tendon in the horse is most commonly injured in superficial digital flexor tendinitis ('bowed tendon')?

Suspensory ligament
Deep digital flexor tendon
Superficial digital flexor tendon (Correct answer)
Common digital extensor tendon

Correct answer: Superficial digital flexor tendon

The superficial digital flexor tendon (SDFT) is the most frequently injured tendon in racehorses, resulting in the classic 'bowed tendon' appearance due to thickening and inflammation.

Question 72: A 3-year-old llama presents with progressive weight loss, bottle jaw, pale mucous membranes, and a fecal egg count of 8,500 Haemonchus contortus eggs per gram. The owner has been rotating between ivermectin and benzimidazoles every 60 days. Fecal egg count reduction tests show <70% reduction with both drug classes. Which anthelmintic regimen is NOW considered the most appropriate next step?

Moxidectin at 5× the standard dose to overcome macrocyclic lactone resistance
Monepantel administered at the label dose for small ruminants
Closantel given orally, leveraging its adulticidal activity against blood-feeding nematodes
Triple combination therapy: ivermectin + levamisole + oxfendazole at standard doses (Correct answer)

Correct answer: Triple combination therapy: ivermectin + levamisole + oxfendazole at standard doses

Multi-drug resistance to both macrocyclic lactones and benzimidazoles is confirmed by the FECRT results. Current evidence-based guidelines for anthelmintic-resistant Haemonchus in camelids and small ruminants recommend triple combination therapy using one drug from each major class (ML + levamisole + benzimidazole) simultaneously, as resistant alleles to all three classes simultaneously are rare. This approach is supported by FAMACHA-guided targeted selective treatment. Monepantel (amino-acetonitrile derivative) is registered in some countries but not currently approved in the US for camelids and efficacy data are limited. High-dose moxidectin does not reliably overcome confirmed resistance and increases toxicity risk. Closantel is not approved in North America and resistance develops rapidly.

Question 73: Fluoroquinolones exert their bactericidal effect by inhibiting which bacterial enzyme?

RNA polymerase beta subunit
Dihydropteroate synthase
DNA gyrase (topoisomerase II) and topoisomerase IV (Correct answer)
Penicillin-binding proteins

Correct answer: DNA gyrase (topoisomerase II) and topoisomerase IV

Fluoroquinolones inhibit bacterial DNA gyrase and topoisomerase IV, enzymes necessary for DNA supercoiling and decatenation, halting DNA replication and causing bacterial cell death.

Question 74: Which mechanism accounts for aspirin's irreversible inhibition of platelet aggregation?

Blockade of GPIIb/IIIa fibrinogen receptor on activated platelets
Phosphodiesterase inhibition raising cAMP and preventing platelet activation
Acetylation of serine residue on COX-1, permanently blocking thromboxane A2 synthesis (Correct answer)
Competitive inhibition of ADP P2Y12 receptor on platelets

Correct answer: Acetylation of serine residue on COX-1, permanently blocking thromboxane A2 synthesis

Aspirin irreversibly acetylates serine-529 of COX-1 in platelets, permanently inhibiting thromboxane A2 production; since platelets lack nuclei, they cannot synthesize new COX-1 for their entire lifespan (~7–10 days).

Question 75: Which MRI sequence is most sensitive for detecting acute ischemic stroke within the first few hours?

Diffusion-weighted imaging (DWI) (Correct answer)
T2-weighted imaging
FLAIR
T1-weighted imaging

Correct answer: Diffusion-weighted imaging (DWI)

DWI detects restricted diffusion within minutes of ischemic stroke onset, far earlier than T2 or FLAIR sequences.

Question 76: On echocardiography, an M-mode finding of paradoxical septal motion is most commonly associated with which condition?

Mitral stenosis
Left ventricular hypertrophy
Right bundle branch block or volume overload of the right ventricle (Correct answer)
Pericardial effusion

Correct answer: Right bundle branch block or volume overload of the right ventricle

Paradoxical septal motion (septal moving anteriorly in systole) occurs with RBBB and right ventricular volume overload conditions like atrial septal defect.

Question 77: A lateral skull X-ray shows a biconvex (lenticular) hyperdense collection. What is the most likely diagnosis on CT head?

Subdural hematoma
Contusion
Subarachnoid hemorrhage
Epidural hematoma (Correct answer)

Correct answer: Epidural hematoma

Epidural hematomas appear as biconvex hyperdense collections because blood accumulates between the dura and inner table of the skull.

Question 78: On flow cytometry of a bone marrow aspirate, a population of cells is identified that is CD19+, CD5+, CD23+, CD20 dim, surface IgM dim, and FMC7−. The Matutes score is 4. A FISH panel reveals del(17p) and del(11q). Which prognostic statement is MOST accurate regarding this patient's disease course?

Del(11q) predicts excellent response to alkylating agents as monotherapy
Del(17p) indicates TP53 dysfunction and poor response to most conventional therapies (Correct answer)
The combination of del(17p) and del(11q) is associated with the best overall survival in CLL
Del(17p) alone confers a favorable prognosis with standard chemoimmunotherapy

Correct answer: Del(17p) indicates TP53 dysfunction and poor response to most conventional therapies

Del(17p) disrupts TP53 and is the most adverse cytogenetic abnormality in CLL, conferring resistance to purine analogues and alkylating agents. These patients have the shortest time to treatment and overall survival. Del(11q) (ATM loss) is also high-risk but responds better to bendamustine-based regimens. BTK inhibitors (ibrutinib) are now preferred for del(17p) CLL. The combination of both deletions is not a favorable prognostic pairing.

Question 79: A 4-year-old intact male cat is presented with a history of siring no offspring despite multiple breedings with proven queens. Semen evaluation reveals 85% morphologically normal spermatozoa with good motility. Which diagnostic step is MOST likely to identify the underlying cause?

Hormonal assay for serum inhibin B as a Sertoli cell marker
Retrograde ejaculation evaluation by post-ejaculatory urinalysis (Correct answer)
Measurement of seminal fructose to assess seminal vesicle function
Testicular fine-needle aspiration to evaluate spermatogenesis

Correct answer: Retrograde ejaculation evaluation by post-ejaculatory urinalysis

Cats lack seminal vesicles, so fructose measurement is not applicable. Retrograde ejaculation — where semen is propelled backward into the bladder rather than anterograde — is a well-documented cause of infertility in tomcats despite normal semen quality. Post-ejaculatory urinalysis (centrifugation and sediment examination for spermatozoa) is the key diagnostic step. Testicular FNA would show normal spermatogenesis given the normal ejaculate parameters, and inhibin B is not routinely used as a primary infertility diagnostic in feline practice.

Question 80: A researcher studies a novel drug that exhibits the following kinetics: doubling the dose quadruples the AUC, and the drug's clearance decreases as plasma concentration increases. This drug most likely exhibits which property?

Time-dependent pharmacokinetics caused by autoinduction of CYP3A4
Enterohepatic recirculation prolonging its half-life
Zero-order (saturation) kinetics due to a capacity-limited elimination process (Correct answer)
First-order elimination with a large volume of distribution

Correct answer: Zero-order (saturation) kinetics due to a capacity-limited elimination process

When doubling the dose more than doubles AUC (here it quadruples it), and clearance decreases with increasing concentration, this is characteristic of saturation (Michaelis-Menten or zero-order) kinetics. The eliminating enzyme or transporter becomes saturated, so a larger fraction of the drug accumulates with each dose increase. Classic examples include phenytoin and ethanol. In pure first-order kinetics, AUC would double proportionally with dose and clearance remains constant.

Question 81: Which regulatory body in the US directly oversees the Animal Welfare Act compliance for research institutions?

FDA Center for Veterinary Medicine
CDC Office of Safety, Security, and Asset Management
NIH Office of Laboratory Animal Welfare
USDA APHIS Animal Care (Correct answer)

Correct answer: USDA APHIS Animal Care

USDA APHIS Animal Care conducts inspections and enforces AWA regulations at research facilities, zoos, and other covered establishments.

Question 82: Which trace mineral deficiency in cattle causes 'falling disease' (sudden cardiac failure) and depigmentation of the hair coat?

Copper deficiency (Correct answer)
Iodine deficiency
Selenium deficiency
Zinc deficiency

Correct answer: Copper deficiency

Copper deficiency impairs cytochrome c oxidase in cardiac muscle (causing heart failure) and tyrosinase activity (causing hair depigmentation and sunbleached coat).

Question 83: In ruminants, the reticulum plays a critical role beyond fermentation. Which anatomical feature of the reticuloruminal fold is most directly responsible for the selective passage of liquid digesta versus large particle matter into the omasum?

The cranial pillar of the rumen generating peristaltic waves that sort digesta by density
The reticulo-omasal orifice, which is guarded by muscular lips that perform size-selective filtration based on particle buoyancy and contractile rhythm (Correct answer)
The reticular groove (sulcus reticuli) contracting to form a tube that bypasses the rumen in neonates
The omasal canal acting as a sieve via its laminae to trap coarse particles before rumination

Correct answer: The reticulo-omasal orifice, which is guarded by muscular lips that perform size-selective filtration based on particle buoyancy and contractile rhythm

The reticulo-omasal orifice is flanked by muscular lips that open and close in coordination with reticular contractions. The biphasic reticular contraction cycle selectively allows small-particle and liquid digesta to pass into the omasum while large particles are retained for further rumination. The omasal laminae do contribute to water absorption downstream, and the reticular groove is real but functions in neonates to shunt milk directly to the abomasum — it is not the primary mechanism for particle-size selection in adults.

Question 84: Carnitine is essential for fatty acid beta-oxidation because it is required for which specific transport step?

Electron transfer in the respiratory chain
Fatty acid activation to acyl-CoA in the cytoplasm
Transfer of long-chain acyl groups across the inner mitochondrial membrane (Correct answer)
NADH regeneration in the matrix

Correct answer: Transfer of long-chain acyl groups across the inner mitochondrial membrane

Carnitine shuttles long-chain fatty acyl groups across the inner mitochondrial membrane via the carnitine shuttle system so beta-oxidation can occur in the matrix.

Question 85: A 10-year-old West Highland White Terrier is presented for a chronic, non-productive cough. Thoracic radiographs reveal prominent, thickened bronchial walls, which appear as 'doughnuts' on end-on views and 'tramlines' when viewed longitudinally. The lung fields appear mildly hyperinflated, but there is no evidence of an alveolar pattern or cardiomegaly. Which of the following is the most likely diagnosis?

Aspiration pneumonia
Cardiogenic pulmonary edema
Pulmonary neoplasia
Chronic bronchitis (Correct answer)

Correct answer: Chronic bronchitis

The radiographic signs described, including thickened bronchial walls seen as 'doughnuts' and 'tramlines', are classic for a bronchial lung pattern. In an older small breed dog with a chronic cough, this pattern is most consistent with chronic bronchitis. Aspiration pneumonia typically presents with an alveolar pattern in the cranioventral lung fields. Cardiogenic edema usually involves an interstitial to alveolar pattern, often in the perihilar or caudodorsal regions, along with cardiomegaly. Pulmonary neoplasia would more likely present as a distinct mass or a structured interstitial (nodular) pattern.

Question 86: A 28-year-old man develops rapidly progressive glomerulonephritis. Renal biopsy shows linear IgG deposits along the glomerular basement membrane. Serum anti-GBM antibodies are positive. What is the diagnosis?

IgA nephropathy
Goodpasture syndrome (Correct answer)
Post-streptococcal glomerulonephritis
Membranous nephropathy

Correct answer: Goodpasture syndrome

Goodpasture syndrome is caused by anti-GBM antibodies (Type II hypersensitivity) producing linear immunofluorescence and rapidly progressive glomerulonephritis often with pulmonary hemorrhage.

Question 87: A patient being treated for heart failure with furosemide, a loop diuretic, is at an increased risk for which of the following electrolyte imbalances?

Hypermagnesemia
Hypercalcemia
Hypokalemia (Correct answer)
Hyperkalemia

Correct answer: Hypokalemia

Loop diuretics, such as furosemide, act on the thick ascending limb of the loop of Henle. They inhibit the Na-K-2Cl cotransporter, leading to increased excretion of sodium, potassium, chloride, calcium, and magnesium. The most significant and common electrolyte disturbance is the loss of potassium, leading to hypokalemia.

Question 88: The IACUC (Institutional Animal Care and Use Committee) is required by US regulations to include which member category?

A representative from the state veterinary licensing board
A licensed pharmacist
At least one non-affiliated community member unrelated to the institution (Correct answer)
A biostatistician

Correct answer: At least one non-affiliated community member unrelated to the institution

Federal regulations require IACUC membership to include at least one veterinarian, one scientist, one non-scientist, and one person unaffiliated with the institution to ensure community perspective.

Question 89: Intraoperatively, a patient develops masseter muscle rigidity after succinylcholine administration. Core temperature is 37.2°C and the patient is hemodynamically stable. Which action is MOST appropriate at this time?

Proceed cautiously — masseter spasm alone is physiologic and requires only observation
Discontinue all triggering agents, monitor creatine kinase, lactate, and temperature closely, and have dantrolene immediately available (Correct answer)
Switch to a non-depolarizing agent and complete the case under total intravenous anesthesia without further intervention
Administer dantrolene 2.5 mg/kg immediately and cancel the case

Correct answer: Discontinue all triggering agents, monitor creatine kinase, lactate, and temperature closely, and have dantrolene immediately available

Isolated masseter muscle rigidity (MMR) after succinylcholine is considered a forme fruste or harbinger of malignant hyperthermia (MH) susceptibility in a significant proportion of cases — estimates range from 15–50%. The current evidence-based approach is to discontinue volatile agents and succinylcholine, convert to TIVA if the case must continue, monitor continuously for MH signs (rising EtCO₂, temperature, acidosis, myoglobinuria), and have dantrolene immediately at bedside. Prophylactic dantrolene without MH signs is not mandatory but must be available. Simply observing (B) underestimates risk; proceeding with TIVA without vigilance (D) ignores the ongoing MH risk window.

Question 90: A cherry eye (prolapse of the nictitating membrane gland) in a young Bulldog should be managed by:

Systemic immunosuppression to reduce gland swelling
Topical antibiotics alone as it often resolves spontaneously
Surgical repositioning (pocket or tacking technique) to preserve glandular tear production (Correct answer)
Surgical excision of the prolapsed gland to prevent recurrence

Correct answer: Surgical repositioning (pocket or tacking technique) to preserve glandular tear production

The third eyelid (nictitating membrane) gland contributes approximately 30–50% of the aqueous portion of the tear film. Excision was historically performed but leads to an unacceptably high rate of keratoconjunctivitis sicca (KCS/dry eye). Current standard of care is surgical repositioning using the 'pocket technique' or anchoring sutures to preserve the gland's tear-producing function. Medical management alone is generally ineffective for established prolapse.

Question 91: A patient on warfarin is started on fluconazole for a fungal infection. Three days later, their INR has risen dangerously. Which mechanism best explains this interaction?

Fluconazole displaces warfarin from albumin binding sites
Fluconazole induces P-glycoprotein, increasing warfarin bioavailability
Fluconazole inhibits CYP2C9, reducing warfarin S-enantiomer metabolism (Correct answer)
Fluconazole inhibits vitamin K epoxide reductase directly

Correct answer: Fluconazole inhibits CYP2C9, reducing warfarin S-enantiomer metabolism

Warfarin's S-enantiomer (the more potent form) is primarily metabolized by CYP2C9. Fluconazole is a potent CYP2C9 inhibitor, which dramatically reduces S-warfarin clearance, leading to elevated plasma concentrations and supratherapeutic anticoagulation. The R-enantiomer is less affected (it is metabolized mainly by CYP3A4/1A2). Albumin displacement is clinically insignificant for warfarin in practice, and fluconazole does not directly inhibit VKOR.

Question 92: A CT scan shows a hypodense area in the right middle cerebral artery territory with loss of the gray-white matter differentiation. This finding is most consistent with:

Cerebral abscess
Multiple sclerosis plaque
Acute ischemic infarct (Correct answer)
Glioblastoma multiforme

Correct answer: Acute ischemic infarct

Loss of gray-white differentiation in an arterial territory on CT indicates acute ischemic infarction due to cytotoxic edema.

Question 93: A 60-year-old woman develops bone pain and hypercalcemia. X-ray shows 'punched-out' lytic lesions. Bone marrow biopsy shows >10% plasma cells. Serum protein electrophoresis shows an M-spike. What is the diagnosis?

Metastatic breast cancer
Multiple myeloma (Correct answer)
Chronic lymphocytic leukemia
Waldenstrom macroglobulinemia

Correct answer: Multiple myeloma

Multiple myeloma presents with CRAB criteria (hypercalcemia, Renal failure, Anemia, Bone lesions) and clonal plasma cells producing a monoclonal immunoglobulin M-spike.

Question 94: A 55-year-old woman is found to have pellagra. Which vitamin deficiency is responsible for this condition?

Pyridoxine (B6)
Cobalamin (B12)
Riboflavin (B2)
Niacin (B3) (Correct answer)

Correct answer: Niacin (B3)

Pellagra is caused by niacin (vitamin B3) deficiency and presents with the classic triad of dermatitis, diarrhea, and dementia.

Question 95: A 28-year-old woman has recurrent miscarriages and thromboses. Laboratory testing shows prolonged aPTT that does not correct with 1:1 mixing with normal plasma. What does failure to correct with mixing suggest?

Presence of an inhibitor (antibody) rather than a factor deficiency (Correct answer)
Vitamin K deficiency
Hemophilia A
Factor VIII deficiency

Correct answer: Presence of an inhibitor (antibody) rather than a factor deficiency

Failure of the prolonged aPTT to correct with normal plasma mixing indicates the presence of an inhibitor (such as lupus anticoagulant), not a factor deficiency.

Question 96: Which antidote is most appropriate for acetaminophen overdose presenting within 8 hours of ingestion?

Naloxone
Fomepizole
N-acetylcysteine (Correct answer)
Flumazenil

Correct answer: N-acetylcysteine

N-acetylcysteine replenishes glutathione stores and acts as a precursor to sulfate, detoxifying the hepatotoxic NAPQI metabolite produced during acetaminophen overdose.

Question 97: In horses, what structure must be manually ruptured or allowed to resolve before Caslick's vulvoplasty repair can be effective for pneumovagina?

Hymen remnant (Correct answer)
Cervical seal
Rectovaginal fistula
Perineal body defect

Correct answer: Hymen remnant

A persistent hymen can trap air or fluid in the vagina; however, Caslick's procedure specifically closes the dorsal vulvar lips to prevent pneumovagina caused by poor perineal conformation.

Question 98: During an outbreak investigation, a cohort study of foodborne illness at a catered event reveals the following: attack rate among those who ate the shrimp = 72%, attack rate among those who did NOT eat the shrimp = 68%. The relative risk is 1.06. What is the MOST appropriate conclusion?

The study is underpowered and a larger cohort is needed before conclusions can be drawn
Shrimp is confirmed as the causative vehicle given the high absolute attack rates
Confounding by another food consumed simultaneously negates the relative risk calculation
Shrimp is unlikely to be the causative vehicle; another food item should be investigated (Correct answer)

Correct answer: Shrimp is unlikely to be the causative vehicle; another food item should be investigated

A relative risk of 1.06 is very close to 1.0, indicating essentially no increased risk from eating shrimp. Despite the high absolute attack rates (suggesting a widespread exposure), the nearly identical rates in both groups means shrimp is not the differentiating factor. Classic outbreak investigation methodology requires identifying a food with a substantially elevated relative risk (typically >2.0) to implicate it as the vehicle. Another food item consumed by nearly everyone — regardless of shrimp consumption — is the more likely culprit.

Question 99: A 28-year-old woman presents with painless cervical lymphadenopathy. Excisional biopsy shows effaced nodal architecture with Reed-Sternberg cells in a background of lymphocytes, plasma cells, and eosinophils with broad bands of fibrosis. EBV-encoded RNA (EBER) in situ hybridization is NEGATIVE. This EBV-negative status is most characteristically associated with which subtype?

Nodular sclerosis classic Hodgkin lymphoma (Correct answer)
Mixed cellularity classic Hodgkin lymphoma
Lymphocyte-depleted classic Hodgkin lymphoma
Lymphocyte-rich classic Hodgkin lymphoma

Correct answer: Nodular sclerosis classic Hodgkin lymphoma

EBV association varies significantly among Hodgkin lymphoma subtypes. Nodular sclerosis (NS-CHL) is the most common subtype in developed countries and is EBV-positive in only ~10–25% of cases. In contrast, mixed cellularity CHL is EBV-positive in ~75% of cases, lymphocyte-depleted CHL is EBV-positive in ~75–96%, and lymphocyte-rich CHL in ~40%. The clinical scenario — young woman, cervical nodes, fibrous bands, lacunar Reed-Sternberg cells — is classic NS-CHL, which is also the subtype LEAST associated with EBV.

Question 100: Which hormone assay is used to diagnose pregnancy in mares after day 60–90 of gestation by detecting placental endocrine secretion?

eCG (equine chorionic gonadotropin) (Correct answer)
Progesterone
Inhibin
Estrone sulfate

Correct answer: eCG (equine chorionic gonadotropin)

eCG (also called PMSG) is produced by the endometrial cups from approximately day 40 to 130–150 of equine gestation and can confirm pregnancy during that window.

Question 101: During a canine parvovirus-2 (CPV-2) outbreak at a shelter, a veterinarian notes that three adult vaccinated dogs seroconverted (showing 4-fold titer rises) but remained completely asymptomatic. Which mechanism MOST accurately explains subclinical infection in these vaccinated adults compared to fatal disease in unvaccinated puppies?

Pre-existing memory B cells in vaccinated adults enable anamnestic IgG responses that neutralize virus before it reaches the rapidly dividing crypt epithelium and bone marrow progenitors (Correct answer)
Maternally derived antibody in puppies paradoxically blocks vaccine-induced immunity while failing to neutralize wild-type virus at mucosal surfaces
Adult dogs express higher levels of transferrin receptor 1 (TfR1), the CPV-2 receptor, allowing faster viral clearance via receptor downregulation
CPV-2 tropism for mitotically active cells is age-independent; the difference in outcome relates solely to gut microbiome composition affecting secondary bacterial invasion

Correct answer: Pre-existing memory B cells in vaccinated adults enable anamnestic IgG responses that neutralize virus before it reaches the rapidly dividing crypt epithelium and bone marrow progenitors

CPV-2 selectively infects mitotically active cells (intestinal crypt epithelium, lymphoid tissue, bone marrow). In vaccinated adults, vaccine-primed memory B and T cells mount a rapid anamnestic response, producing neutralizing IgG within 24–48 hours of exposure — before viral titers reach levels sufficient to deplete the crypt stem cell population. Unvaccinated puppies lack this memory response; their naïve immune system responds too slowly, allowing massive viral replication in the crypt epithelium, leading to villous blunting, protein-losing enteropathy, and bacteremia.

Question 102: A 55-year-old male presents to the emergency department with altered mental status, visual disturbances, and severe metabolic acidosis with an elevated anion gap. History from a family member suggests he may have ingested windshield wiper fluid. Which of the following is the most appropriate initial antidote?

Acetylcysteine
Fomepizole (Correct answer)
Naloxone
Sodium Bicarbonate

Correct answer: Fomepizole

The patient's symptoms are classic for methanol poisoning, a primary component of windshield wiper fluid. Methanol is metabolized by alcohol dehydrogenase to toxic metabolites like formic acid, which causes metabolic acidosis and optic nerve damage. Fomepizole is a potent inhibitor of alcohol dehydrogenase and is the preferred antidote to prevent the formation of these toxic metabolites. While sodium bicarbonate may be used to correct acidosis, it is not the primary antidote. Naloxone is for opioid overdose, and acetylcysteine is for acetaminophen toxicity.

Question 103: On a chest X-ray, Kerley B lines appear as short horizontal lines at the lung periphery and indicate which underlying process?

Pneumothorax
Pulmonary embolism
Pleural effusion
Pulmonary interstitial edema causing lymphatic distension (Correct answer)

Correct answer: Pulmonary interstitial edema causing lymphatic distension

Kerley B lines represent edema in the interlobular septa due to elevated pulmonary venous pressure, characteristic of interstitial pulmonary edema.

Question 104: A 2-year-old Cocker Spaniel has chronic bilateral otitis externa with secondary Malassezia overgrowth. Dermatology suspects an underlying etiology. Which is the most likely primary cause?

Hypothyroidism
Atopic dermatitis (Correct answer)
Sarcoptic mange
Food allergy

Correct answer: Atopic dermatitis

Atopic dermatitis is the most common primary cause of recurrent Malassezia otitis in dogs, with Cocker Spaniels being a predisposed breed.

Question 105: A 45-year-old man undergoes a radical parotidectomy. Postoperatively he develops Frey syndrome — gustatory sweating over the preauricular skin when eating. Which embryological misrouting best explains this phenomenon?

Aberrant regeneration of the auriculotemporal nerve fibers into sympathetic sweat gland pathways (Correct answer)
Regeneration of chorda tympani fibers into the lingual sweat glands
Collateral sprouting of the lesser petrosal nerve into somatic sensory pathways
Cross-innervation of the great auricular nerve with the facial nerve

Correct answer: Aberrant regeneration of the auriculotemporal nerve fibers into sympathetic sweat gland pathways

Frey syndrome results when postganglionic parasympathetic secretomotor fibers (originally destined for the parotid gland via the auriculotemporal nerve from the otic ganglion) aberrantly reinnervate the sweat glands and cutaneous vasculature of the preauricular skin, which are normally under sympathetic control. Eating stimulates parasympathetic activity that now triggers sweating rather than salivation.

Question 106: A serum protein electrophoresis shows a sharp, narrow peak in the gamma region. The next best step to characterize this finding is:

Serum immunofixation electrophoresis to identify the heavy and light chain class (Correct answer)
Serum uric acid measurement
Bone scan
Repeat CBC in 3 months

Correct answer: Serum immunofixation electrophoresis to identify the heavy and light chain class

Immunofixation electrophoresis identifies and characterizes the specific immunoglobulin class (e.g., IgG kappa) of an M-spike detected on SPEP.

Question 107: A critically ill ICU patient on enteral nutrition develops diarrhea, hyponatremia, and muscle cramps. Serum levels reveal the likely cause. Which electrolyte is most likely depleted?

Potassium
Zinc
Magnesium (Correct answer)
Selenium

Correct answer: Magnesium

Hypomagnesemia in ICU patients causes refractory hypokalemia and hyponatremia, muscle cramps, and arrhythmias, and is common with diarrhea-associated losses.

Question 108: Which bone of the equine forelimb is commonly called the 'coffin bone' and is enclosed within the hoof capsule?

Navicular bone
Middle phalanx (P2)
Distal phalanx (P3) (Correct answer)
Proximal phalanx (P1)

Correct answer: Distal phalanx (P3)

The distal phalanx (P3, pedal bone) is the third phalanx, entirely enclosed within the hoof capsule and the primary bone affected in laminitis.

Question 109: On ultrasound, which finding is most diagnostic of a testicular torsion?

Absent or markedly decreased Doppler flow in the affected testis (Correct answer)
Increased Doppler flow compared to the contralateral side
Hyperechoic testicular parenchyma
Hydrocele without parenchymal changes

Correct answer: Absent or markedly decreased Doppler flow in the affected testis

Absent or significantly decreased intratesticular Doppler flow confirms testicular torsion and requires emergent surgical intervention.

Question 110: Allogrooming in social species such as cats and non-human primates primarily functions as:

Dominance assertion
Territorial marking
Parasite removal only
Social bonding and tension reduction (Correct answer)

Correct answer: Social bonding and tension reduction

While allogrooming does remove ectoparasites, its primary adaptive function in most social species is reinforcing affiliative bonds and reducing social tension.

Question 111: Under the Russell and Burch 3Rs framework, which scenario represents the application of Refinement at the highest level of implementation priority, as subsequently elaborated by Balls et al. (1995)?

Adopting continuous telemetric monitoring to detect pain endpoints earlier and implement humane endpoints sooner (Correct answer)
Replacing a rodent model of surgical pain with an in vitro nociceptor assay
Reducing the statistical group size from n=20 to n=12 through power analysis before study initiation
Using isoflurane rather than ketamine/xylamine for anesthesia induction to shorten recovery time

Correct answer: Adopting continuous telemetric monitoring to detect pain endpoints earlier and implement humane endpoints sooner

Balls et al. (1995) stratified Refinement into three levels: (1) improvements to husbandry and care, (2) improvements to experimental procedures, and (3) implementation of humane endpoints. Early humane endpoints—enabled here by continuous telemetric monitoring that provides objective, real-time physiological data—represent the highest-priority Refinement intervention because they directly reduce the severity and duration of suffering. Replacing a model is Replacement (not Refinement). Reducing n is Reduction. Anesthetic choice (isoflurane vs. ketamine) improves procedural welfare (level 2 Refinement) but does not address the critical welfare gain of earlier humane endpoint detection that continuous telemetry affords.

Question 112: A dog undergoing orthopedic surgery develops shivering and a rectal temperature of 35.5°C (95.9°F). Which intraoperative measure is MOST effective in preventing further heat loss?

Place ice packs around the neck
Increase the anesthetic gas concentration
Use forced-air warming blankets and warm IV fluids (Correct answer)
Administer IV dexamethasone

Correct answer: Use forced-air warming blankets and warm IV fluids

Forced-air warming blankets are the most effective active rewarming modality during surgery, and warming IV fluids prevents further core temperature loss.

Question 113: Ammonia toxicity in ruminants fed high non-protein nitrogen (NPN) diets occurs because excess ammonia overwhelms the liver's ability to convert it to urea via the urea cycle. Which enzyme catalyzes the rate-limiting, first committed step of the urea cycle?

Argininosuccinate lyase
Carbamoyl phosphate synthetase I (CPS I) (Correct answer)
Ornithine transcarbamylase
Arginase

Correct answer: Carbamoyl phosphate synthetase I (CPS I)

Carbamoyl phosphate synthetase I (CPS I), located in the mitochondrial matrix, catalyzes the ATP-dependent conversion of ammonia + CO2 → carbamoyl phosphate. This is the rate-limiting step of the hepatic urea cycle and is allosterically activated by N-acetylglutamate. When ammonia influx exceeds CPS I capacity (as with rumen NPN overload), blood ammonia rises, causing neurological signs.

Question 114: During a splenectomy, which vessel must be carefully identified and ligated to avoid inadvertent injury to the pancreas?

Right gastroepiploic artery
Celiac artery
Short gastric vessels and left gastroepiploic artery near the splenic hilus (Correct answer)
Left gastric artery

Correct answer: Short gastric vessels and left gastroepiploic artery near the splenic hilus

The short gastric vessels and left gastroepiploic artery run near the splenic hilus in close proximity to the pancreatic tail, requiring careful dissection to avoid pancreatitis.

Question 115: In horses, the 'stay apparatus' of the hindlimb locks the stifle and hock in which position, allowing standing sleep?

Extended (Correct answer)
Abducted
Adducted
Flexed

Correct answer: Extended

The reciprocal apparatus and stay apparatus lock the stifle and hock joints in extension with minimal muscular effort, enabling horses to sleep standing up.

Question 116: A toxicology fellow reviews a case of suspected digoxin toxicity. The patient's ECG shows frequent PVCs in a bigeminal pattern, and serum potassium is 2.9 mEq/L. Why does hypokalemia exacerbate digoxin toxicity at the molecular level?

Hypokalemia activates protein kinase A, which phosphorylates the Na⁺/K⁺-ATPase, making it resistant to digoxin
Low extracellular potassium reduces the resting membrane potential of cardiomyocytes, making them more susceptible to digoxin-induced triggered activity
Potassium and digoxin compete for the same extracellular binding site on the α-subunit of Na⁺/K⁺-ATPase; hypokalemia increases digoxin binding affinity to the pump (Correct answer)
Hypokalemia increases digoxin renal clearance, paradoxically raising free digoxin levels by reducing tubular reabsorption competition

Correct answer: Potassium and digoxin compete for the same extracellular binding site on the α-subunit of Na⁺/K⁺-ATPase; hypokalemia increases digoxin binding affinity to the pump

Digoxin exerts its effect by binding to the extracellular face of the α-subunit of Na⁺/K⁺-ATPase at the same site where K⁺ binds during normal pump cycling. K⁺ and digoxin are competitive at this binding site—when extracellular K⁺ is low (hypokalemia), there is less competition, and digoxin binds the pump more avidly and for longer periods, dramatically enhancing Na⁺/K⁺-ATPase inhibition. This leads to greater intracellular Na⁺ accumulation, reduced Ca²⁺ extrusion via NCX, elevated intracellular Ca²⁺, and increased risk of delayed afterdepolarizations and triggered arrhythmias. This is why potassium repletion is a cornerstone of digoxin toxicity management.

Question 117: A 7-year-old DSH cat with chronic vomiting and weight loss has a PLI within normal limits but shows small intestinal wall thickening on ultrasound with loss of layering. The most likely diagnosis is:

Alimentary lymphoma (Correct answer)
Tritrichomonas foetus infection
Chronic pancreatitis
Inflammatory bowel disease

Correct answer: Alimentary lymphoma

Loss of normal intestinal wall layering on ultrasound in a middle-aged to older cat with chronic GI signs strongly suggests alimentary lymphoma.

Question 118: Which pulmonary function test finding best predicts post-operative pulmonary complications?

DLCO < 60% predicted
FEV1/FVC ratio < 0.70
FEV1 < 40% predicted (Correct answer)
FVC < 80% predicted

Correct answer: FEV1 < 40% predicted

An FEV1 < 40% predicted indicates severe obstructive disease and is associated with significantly increased risk of post-operative respiratory complications.

Question 119: A 5-year-old female spayed Labrador Retriever is referred for refractory inflammatory bowel disease. She has been on prednisolone 2 mg/kg/day for 8 weeks with minimal response. Endoscopic biopsies show diffuse mucosal eosinophilic infiltration with fibrosis extending into the submucosa, and tissue culture grows Pythium insidiosum. Serology for Pythium-specific ELISA is strongly positive. Which treatment approach is best supported by current evidence?

Initiate amphotericin B liposomal formulation IV at 3 mg/kg q48h for 12 doses, as Pythium is an oomycete with ergosterol in its cell wall making it highly susceptible to polyene antifungals.
Discontinue prednisolone, start voriconazole monotherapy at 4 mg/kg PO BID, and repeat endoscopy in 6 weeks; voriconazole has the highest minimum inhibitory concentration activity against Pythium among azoles.
Add itraconazole 5 mg/kg PO BID combined with terbinafine 5–10 mg/kg PO daily, as this azole-allylamine combination has the best documented clinical response rate for gastrointestinal pythiosis in dogs, alongside surgical resection of affected bowel if focal. (Correct answer)
Immunotherapy using Pythium insidiosum-specific antigen vaccine combined with itraconazole is the first-line approach given the immunological basis of oomycete control in dogs.

Correct answer: Add itraconazole 5 mg/kg PO BID combined with terbinafine 5–10 mg/kg PO daily, as this azole-allylamine combination has the best documented clinical response rate for gastrointestinal pythiosis in dogs, alongside surgical resection of affected bowel if focal.

Pythium insidiosum is an oomycete (water mold), not a true fungus — its cell membrane contains cellulose and non-ergosterol sterols, making it intrinsically resistant to amphotericin B and most azoles. However, the combination of itraconazole and terbinafine has shown the best clinical outcomes in canine gastrointestinal pythiosis by targeting multiple steps in sterol synthesis pathways. Surgical resection of focal lesions combined with medical therapy improves survival. Voriconazole has poor activity against Pythium despite good azole spectrum otherwise. Immunotherapy (Pythium vaccine) exists but is not established as first-line monotherapy and is used as an adjunct.

Question 120: A dog presenting with dilated cardiomyopathy is placed on a grain-free diet; which nutritional deficiency has been epidemiologically associated with this presentation?

Excess dietary sodium
Omega-3 fatty acid excess
Taurine deficiency linked to legume-heavy, grain-free formulations (Correct answer)
Magnesium deficiency

Correct answer: Taurine deficiency linked to legume-heavy, grain-free formulations

The FDA investigated a link between grain-free diets high in legumes and taurine-deficiency dilated cardiomyopathy in dogs, particularly golden retrievers.

Question 121: A 9-year-old cat presents with progressive weight loss, polyphagia, and a palpable ventral neck mass. T4 is markedly elevated. After confirming hyperthyroidism, which treatment option provides the best chance of permanent cure?

Radioactive iodine (I-131) therapy (Correct answer)
Dietary iodine restriction alone
Oral methimazole for life
Bilateral thyroidectomy

Correct answer: Radioactive iodine (I-131) therapy

Radioactive iodine (I-131) is considered the gold standard curative treatment for feline hyperthyroidism with >95% cure rate and minimal complications.

Question 122: In a patient with short bowel syndrome retaining only 80 cm of jejunum with an intact ileocecal valve and colon, which adaptation mechanism contributes MOST to long-term enteral autonomy?

Increased gastric acid secretion enhancing proximal absorption
Colonic fermentation of unabsorbed carbohydrates to short-chain fatty acids (Correct answer)
Villous hypertrophy in the remaining jejunum
Upregulation of jejunal SGLT-1 transporter density

Correct answer: Colonic fermentation of unabsorbed carbohydrates to short-chain fatty acids

With an intact colon, colonic bacteria ferment unabsorbed complex carbohydrates into short-chain fatty acids (butyrate, propionate, acetate), which can provide up to 500–1000 kcal/day and are readily absorbed by colonocytes. This colonic energy salvage pathway is the dominant adaptation that enables enteral autonomy in patients retaining their colon, making dietary optimization toward complex carbohydrates a key therapeutic strategy.

Question 123: Environmental enrichment for non-human primates in research settings primarily serves to:

Promote species-typical behavior and reduce psychological distress (Correct answer)
Simplify husbandry procedures
Increase caloric intake
Reduce aggression between technicians and animals

Correct answer: Promote species-typical behavior and reduce psychological distress

Enrichment provides cognitive stimulation and social outlets that allow expression of species-typical behaviors, directly improving psychological welfare.

Question 124: Which suture pattern is most appropriate for closing a hollow viscus such as the intestine to achieve an inverting, leak-resistant anastomosis?

Simple interrupted
Ford interlocking
Lembert (Correct answer)
Cruciate

Correct answer: Lembert

The Lembert suture pattern is an inverting pattern commonly used for gastrointestinal surgery to create a watertight, serosa-to-serosa seal.

Question 125: A cat develops feline infectious peritonitis (FIP). The pathogenesis involves mutation of feline enteric coronavirus (FECV) to feline infectious peritonitis virus (FIPV). Which molecular change is most consistently associated with this phenotypic conversion?

Acquisition of a novel envelope (E) protein from a recombination event with canine coronavirus
Point mutations in the spike protein receptor-binding domain that shift tropism from enterocytes to macrophages
Methylation-induced silencing of the N protein gene, reducing interferon antagonism
Deletion in the 3c gene leading to altered macrophage tropism (Correct answer)

Correct answer: Deletion in the 3c gene leading to altered macrophage tropism

The conversion from FECV to FIPV most consistently involves mutations or deletions in the 3c gene (and sometimes the 7b gene). Loss-of-function mutations in 3c are strongly correlated with the shift from enteric replication to systemic macrophage-tropic infection, the hallmark of FIPV. While spike protein mutations affect receptor affinity, the 3c gene alteration is the most consistently identified genetic correlate of virulent conversion in sequencing studies. Recombination with CCoV has been documented but is not the dominant mechanism of FIP conversion.

Question 126: A patient undergoes a nuclear medicine bone scan and demonstrates a 'superscan' pattern. Which of the following conditions is LEAST likely to produce this finding?

Multiple myeloma with purely lytic lesions and no osteoblastic response (Correct answer)
Widespread osteoblastic metastases from prostate carcinoma
Diffuse Paget disease involving the axial skeleton
Renal osteodystrophy with secondary hyperparathyroidism

Correct answer: Multiple myeloma with purely lytic lesions and no osteoblastic response

A superscan on bone scintigraphy shows diffusely increased skeletal uptake with absent or markedly reduced renal and soft-tissue activity, creating an apparently 'normal' but abnormally intense image. It occurs when osteoblastic activity is globally elevated — as in widespread osteoblastic metastases (e.g., prostate cancer), metabolic bone disease (renal osteodystrophy, hyperparathyroidism), or diffuse Paget disease. Multiple myeloma classically produces purely lytic lesions that stimulate no osteoblastic response; consequently, bone scan is often falsely negative and a superscan pattern would not be expected.

Question 127: Of the following, which one is not a component of the carpus (wrist)?

Scaphoid
Radius
Lunate
Ulna (Correct answer)

Correct answer: Ulna

The ulna is a forearm bone, not a component of the wrist (carpus).

Question 128: A horse undergoing general anesthesia in dorsal recumbency is at risk for which muscle condition that can result in postoperative myopathy?

Hypokalemic periodic paralysis
Rhabdomyolysis due to prolonged pressure on muscle groups (Correct answer)
Malignant hyperthermia
Tetany due to hypocalcemia

Correct answer: Rhabdomyolysis due to prolonged pressure on muscle groups

Horses in dorsal recumbency experience compression of large muscle groups, reducing perfusion and causing ischemic myopathy (postanesthetic myopathy) if prolonged.

Question 129: The greater omentum in dogs is a peritoneal fold that primarily originates from which organ?

Greater curvature of the stomach (Correct answer)
Spleen
Descending duodenum
Liver

Correct answer: Greater curvature of the stomach

The greater omentum arises from the greater curvature of the stomach and drapes over the intestines, playing roles in fat storage and immune defense.

Question 130: A 4-year-old llama presents with a 6-week history of progressive ascending hindlimb ataxia that has now involved the forelimbs. CSF analysis: protein 85 mg/dL (normal <40), WBC 3 cells/μL, xanthochromia absent. MRI of the cervicothoracic junction shows intramedullary T2 hyperintensity without gadolinium enhancement. Serum and CSF Parelaphostrongylus tenuis ELISA are negative. Thiamine levels are normal. Which diagnosis BEST fits this presentation in a South American camelid?

Cervical vertebral compressive myelopathy (Wobbler syndrome) with subclinical disc protrusion
Protozoal myeloencephalitis caused by Sarcocystis neurona with atypical CSF profile
Camelid spinal cord disease (CSCD) due to aberrant Parelaphostrongylus odocoilei migration, seronegative due to poor antibody cross-reactivity with P. tenuis antigen (Correct answer)
Polioencephalomalacia caused by subclinical thiamine deficiency undetected by standard serum assay

Correct answer: Camelid spinal cord disease (CSCD) due to aberrant Parelaphostrongylus odocoilei migration, seronegative due to poor antibody cross-reactivity with P. tenuis antigen

South American camelids (llamas, alpacas) are aberrant hosts for multiple Parelaphostrongylus species. P. odocoilei causes spinal cord disease in camelids with ascending myelopathy, whereas P. tenuis more commonly causes cerebrocortical signs. Critically, the standard P. tenuis ELISA has poor sensitivity for P. odocoilei infection due to limited antigenic cross-reactivity, producing false-negative serology. The CSF shows elevated protein with minimal pleocytosis (albuminocytologic dissociation), consistent with migration-associated myelopathy. Cervical compressive myelopathy would show gadolinium enhancement at the compression site. Thiamine deficiency causes cortical lesions with gadolinium-enhancing necrosis. S. neurona is rare in camelids and usually shows more pronounced pleocytosis.

Question 131: A patient presents with pinpoint pupils, coma, and respiratory depression after suspected heroin overdose. What is the correct first-line management?

Intravenous flumazenil
Intravenous naloxone (Correct answer)
Activated charcoal via nasogastric tube
Intravenous physostigmine

Correct answer: Intravenous naloxone

Naloxone is a competitive mu-opioid receptor antagonist that rapidly reverses opioid-induced CNS and respiratory depression; its short half-life may require repeated dosing for long-acting opioids.

Question 132: Which pharmacological class is most commonly used to provide pre-emptive multimodal analgesia in veterinary surgical patients to improve welfare?

Opioids combined with NSAIDs (Correct answer)
Alpha-2 agonists as the sole agent
Benzodiazepines alone
Corticosteroids

Correct answer: Opioids combined with NSAIDs

Pre-emptive multimodal analgesia combining opioids (for central analgesia) with NSAIDs (for peripheral/inflammatory pain) is the evidence-based standard for minimizing perioperative pain.

Question 133: The avian renal portal system is physiologically unique among domestic species. Which of the following correctly describes how this system can confound drug pharmacokinetics in birds?

Drugs injected into the caudal half of the body may undergo first-pass hepatic metabolism before reaching systemic circulation because portal blood drains directly into the liver before the caudal vena cava
Drugs injected into the caudal half of the body may be delivered directly to the renal tubular cells and undergo renal extraction or secretion before reaching systemic circulation, reducing bioavailability (Correct answer)
Drugs injected cranial to the synsacrum are subject to renal portal filtration because the renal portal vein receives blood from the jugular anastomosis
The avian renal portal system shunts blood away from the kidneys during digestion, so post-prandial drug injections in the caudal body have enhanced systemic bioavailability

Correct answer: Drugs injected into the caudal half of the body may be delivered directly to the renal tubular cells and undergo renal extraction or secretion before reaching systemic circulation, reducing bioavailability

In birds, the renal portal system carries blood from the caudal limbs and tail through the afferent renal portal veins directly to the peritubular capillaries of the kidney — before that blood returns to the heart via the caudal renal veins and caudal vena cava. Drugs (especially those with significant renal tubular secretion, e.g., penicillins, aminoglycosides) injected into the pectoral muscles or anywhere cranial to the synsacrum bypass this system. Injections into the leg or caudal body may be subject to renal extraction, effectively lowering systemic drug levels. The first-pass effect described in option A is hepatic, not what the renal portal system does.

Question 134: Which virus causes Marek's disease in poultry and belongs to which family?

Avian leukosis virus, family Retroviridae
Infectious bursal disease virus, family Birnaviridae
Gallid herpesvirus 2, family Herpesviridae (Correct answer)
Newcastle disease virus, family Paramyxoviridae

Correct answer: Gallid herpesvirus 2, family Herpesviridae

Marek's disease is caused by Gallid herpesvirus 2 (a cell-associated alphaherpesvirus), which induces T-cell lymphomas in susceptible chickens.

Question 135: During one-lung ventilation for thoracic surgery, hypoxia develops. What is the FIRST intervention?

Apply CPAP (5 cmH2O) to the non-ventilated lung (Correct answer)
Clamp the pulmonary artery to the operative lung
Increase tidal volume on the ventilated lung
Convert to two-lung ventilation immediately

Correct answer: Apply CPAP (5 cmH2O) to the non-ventilated lung

Applying low-level CPAP (5 cmH2O) with 100% O2 to the non-ventilated lung recruits alveoli and improves oxygenation without eliminating the surgical field.

Question 136: On thoracic radiographs, an alveolar pattern with an air bronchogram and cranioventral distribution in a febrile dog with cough is most consistent with:

Aspiration or bacterial bronchopneumonia (Correct answer)
Cardiogenic pulmonary edema
Pulmonary contusions
Primary lung neoplasia

Correct answer: Aspiration or bacterial bronchopneumonia

Bacterial or aspiration pneumonia characteristically produces a cranioventral alveolar pattern with air bronchograms due to gravity-dependent distribution.

Question 137: What is the primary advantage of using a laryngeal mask airway (LMA) over endotracheal intubation in appropriate cases?

Requires less anesthetic depth to place (Correct answer)
Eliminates the need for muscle relaxants
Provides superior protection against aspiration
Enables positive pressure ventilation at higher pressures

Correct answer: Requires less anesthetic depth to place

LMA placement requires less anesthetic depth and blunts hemodynamic responses less than laryngoscopy and intubation.

Question 138: A 9-year-old Hereford bull presents with a 4-month history of bilateral nasal discharge, progressive dyspnea, and weight loss. Endoscopy reveals a cauliflower-like mass at the ethmoid region. Histopathology shows neoplastic cells with characteristic 'Feulgen-positive' intranuclear inclusions and syncytia formation. PCR confirms enzootic nasal tumor virus (ENTV-2). Which retroviral mechanism drives autonomous tumor growth in affected epithelial cells?

Downregulation of MHC class I expression by viral US11 protein preventing cytotoxic T-cell recognition
Autocrine stimulation via viral envelope glycoprotein binding to host Jaagsiekte sheep retrovirus (JSRV) receptor Hyal2
Insertional activation of host proto-oncogenes by the ENTV-2 provirus long terminal repeat (LTR) enhancer elements (Correct answer)
Viral oncogene (v-onc) transduction encoding a constitutively active receptor tyrosine kinase homolog

Correct answer: Insertional activation of host proto-oncogenes by the ENTV-2 provirus long terminal repeat (LTR) enhancer elements

ENTV-2 (Enzootic Nasal Tumor Virus) is a betaretrovirus that, like JSRV in pulmonary adenomatosis, lacks classical viral oncogenes. Transformation occurs through insertional mutagenesis — the proviral LTR contains potent enhancer elements that activate adjacent host proto-oncogenes (particularly Ras and PI3K pathway genes) upon integration. The viral envelope protein itself also has transforming activity via receptor tyrosine kinase signaling, but the primary driver at the population level is LTR enhancer-mediated insertional activation. ENTV does not carry transduced v-onc genes, and the US11 mechanism is herpesviral.

Question 139: A public health investigation is launched after several individuals report severe, sometimes bloody, diarrhea after attending a barbecue. The investigation traces the outbreak to undercooked chicken. Which bacterial pathogen is a leading cause of foodborne gastroenteritis and is most commonly associated with consumption of contaminated poultry?

Clostridium botulinum
Brucella abortus
Listeria monocytogenes
Campylobacter jejuni (Correct answer)

Correct answer: Campylobacter jejuni

Campylobacter jejuni is one of the most common causes of bacterial foodborne illness in the United States and worldwide. Poultry is a major reservoir for the bacteria, and transmission to humans frequently occurs through handling raw poultry or consuming undercooked poultry products.

Question 140: Which of the following correctly pairs a drug with its mechanism of causing QT prolongation via cardiac ion channel blockade?

Ibutilide — blockade of the L-type calcium current (ICaL) with secondary IKr inhibition
Sotalol — blockade of the inward rectifier potassium current (IK1 encoded by KCNJ2)
Dofetilide — blockade of the rapid delayed rectifier potassium current (IKr encoded by hERG) (Correct answer)
Amiodarone — blockade of the slow delayed rectifier potassium current (IKs encoded by KCNQ1/KCNE1)

Correct answer: Dofetilide — blockade of the rapid delayed rectifier potassium current (IKr encoded by hERG)

Dofetilide is a selective IKr blocker — it specifically targets the rapid component of the delayed rectifier potassium current, which is encoded by the hERG (KCNH2) gene. Blocking IKr delays ventricular repolarization, prolonging the QT interval and increasing the risk of torsades de pointes. Amiodarone broadly blocks multiple ion channels (IKr, IKs, INa, ICaL) rather than selectively IKs. Sotalol also primarily blocks IKr, not IK1. Ibutilide's primary mechanism is activation of a slow inward sodium current, not direct ICaL blockade.

Question 141: On ultrasound, which characteristic of a thyroid nodule most strongly indicates the need for fine-needle aspiration biopsy?

A nodule with a peripheral halo and isoechoic texture
Spongiform appearance with small cystic spaces
A purely cystic nodule with posterior acoustic enhancement
Microcalcifications within a solid hypoechoic nodule >1 cm (Correct answer)

Correct answer: Microcalcifications within a solid hypoechoic nodule >1 cm

Microcalcifications in a solid hypoechoic nodule >1 cm are high-risk features per ACR TI-RADS and warrant fine-needle aspiration.

Question 142: Which monitoring modality is most useful for detecting intraoperative myocardial ischemia?

Transesophageal echocardiography wall motion analysis
Lead II ECG only
Pulmonary artery catheter
Five-lead ECG with continuous ST-segment monitoring (leads II and V5) (Correct answer)

Correct answer: Five-lead ECG with continuous ST-segment monitoring (leads II and V5)

Combined monitoring of leads II and V5 detects approximately 96% of intraoperative ST-segment changes, covering both inferior and anterior/lateral territory.

Question 143: A dog is diagnosed with a ruptured cranial cruciate ligament (CCL). The instability caused by this injury, known as a positive cranial drawer sign, is due to the loss of the ligament's primary function. What is the primary function of the CCL in the canine stifle?

To prevent the femur from slipping caudally on the tibial plateau
To prevent hyperextension and limit internal rotation of the tibia (Correct answer)
To provide shock absorption and distribute load within the joint
To prevent abduction or varus angulation of the stifle

Correct answer: To prevent hyperextension and limit internal rotation of the tibia

The cranial cruciate ligament's main role is to prevent cranial translocation (forward movement) of the tibia relative to the femur. It also plays a crucial role in preventing hyperextension of the stifle joint and limiting internal rotation of the tibia. Preventing varus/valgus angulation is the primary role of the collateral ligaments. Preventing caudal slippage is the function of the caudal cruciate ligament, and shock absorption is a primary function of the menisci.

Question 144: The MAC (minimum alveolar concentration) of a volatile anesthetic is defined as the concentration that prevents movement in response to surgical stimulus in what percentage of patients?

75%
95%
50% (Correct answer)
99%

Correct answer: 50%

MAC is the ED50 — the alveolar concentration preventing movement in 50% of patients exposed to a standard surgical stimulus (skin incision).

Question 145: Which vitamin deficiency causes 'rickets' in young animals, and what is the biochemical mechanism?

Vitamin D deficiency impairs intestinal calcium and phosphorus absorption, preventing normal bone mineralization (Correct answer)
Vitamin C deficiency impairs collagen cross-linking in osteoid
Vitamin K deficiency prevents osteocalcin carboxylation
Vitamin A excess causes premature growth plate closure

Correct answer: Vitamin D deficiency impairs intestinal calcium and phosphorus absorption, preventing normal bone mineralization

Vitamin D (as 1,25-dihydroxycholecalciferol) upregulates intestinal calcium-binding protein; deficiency reduces Ca/P absorption, leaving osteoid unmineralized.

Question 146: A 58-year-old male presents with progressive back pain. Plain radiographs reveal vertebral body end-plate irregularities at L3-L4 with loss of disc height and paradoxical increased signal on T2-weighted MRI within the adjacent vertebral bodies. Which of the following is the MOST likely diagnosis?

Schmorl's nodes with reactive edema
Modic Type I degenerative change
Modic Type II fatty degeneration
Septic discitis (Correct answer)

Correct answer: Septic discitis

Septic discitis classically presents with disc space loss and end-plate irregularity on plain film combined with T2 hyperintensity in both the disc and adjacent vertebral bodies on MRI — often indistinguishable from Modic Type I early on, but the 'paradoxical' pairing of plain film destruction with florid T2 signal in the vertebral bodies (not just the disc) is a red flag for infection. Modic Type I shows low T1/high T2 in the end plates due to edema/inflammation but lacks the aggressive end-plate erosion pattern. Modic Type II is low T1 (wait — actually high T1)/high T2 fatty marrow change, a chronic finding without destructive changes.

Question 147: Which mechanism allows Staphylococcus aureus to resist beta-lactam antibiotics such as methicillin (MRSA) in veterinary patients?

Acquisition of mecA gene encoding a modified penicillin-binding protein (PBP2a) with low antibiotic affinity (Correct answer)
Reduced outer membrane permeability preventing antibiotic entry
Efflux pumps that expel beta-lactam antibiotics from the bacterial cell
Production of extended-spectrum beta-lactamase (ESBL) that hydrolyzes the beta-lactam ring

Correct answer: Acquisition of mecA gene encoding a modified penicillin-binding protein (PBP2a) with low antibiotic affinity

The mecA gene encodes PBP2a, an altered penicillin-binding protein with very low affinity for all beta-lactam antibiotics, conferring broad resistance in MRSA strains.

Question 148: A dog is presented with a large, heavily contaminated wound on its flank that is 12 hours old. The veterinarian debrides the wound and places a bandage, planning to surgically close it in 4 days once granulation tissue has formed and infection is controlled. What is this type of wound closure called?

Primary closure
Second intention healing
Secondary closure (Correct answer)
Delayed primary closure

Correct answer: Secondary closure

Secondary closure (or tertiary intention healing) refers to the surgical closure of a wound 3-5 days or more after the initial injury, after a healthy bed of granulation tissue has formed. This is distinct from delayed primary closure (closure within 1-3 days, before granulation tissue forms) and second intention healing (allowing the wound to granulate and epithelialize on its own without surgical closure). Primary closure is the immediate suturing of a clean wound.

Question 149: In health services research, 'effectiveness' differs from 'efficacy' in that effectiveness measures:

Cost per quality-adjusted life year
Outcomes under real-world conditions (Correct answer)
The maximum possible benefit of an intervention
Outcomes under ideal controlled conditions

Correct answer: Outcomes under real-world conditions

Effectiveness measures how well an intervention works under real-world conditions, while efficacy measures performance under ideal trial conditions.

Question 150: In calculating ruminant energy rations, which unit system is used to express energy requirements and feed energy values?

Net energy (NE) expressed in megacalories (Mcal), subdivided into NE for maintenance (NEm), gain (NEg), and lactation (NEl) (Correct answer)
Digestible energy (DE) in kilocalories only
Metabolizable energy (ME) in kilojoules only
Total digestible nutrients (TDN) expressed as a percentage only

Correct answer: Net energy (NE) expressed in megacalories (Mcal), subdivided into NE for maintenance (NEm), gain (NEg), and lactation (NEl)

Ruminant ration balancing uses net energy subdivisions (NEm, NEg, NEl) in Mcal because different physiologic processes use available energy with different efficiencies.

Question 151: Which of the following best describes the primary mechanism of toxicity for organophosphate insecticides?

Irreversible inhibition of acetylcholinesterase (Correct answer)
Uncoupling of oxidative phosphorylation
Blockade of GABA receptors in the CNS
Inhibition of cytochrome P450 enzymes

Correct answer: Irreversible inhibition of acetylcholinesterase

Organophosphates exert their toxic effects by phosphorylating and thereby irreversibly inhibiting acetylcholinesterase (AChE). This enzyme is responsible for breaking down the neurotransmitter acetylcholine. Inhibition of AChE leads to an accumulation of acetylcholine at nerve synapses, causing overstimulation of cholinergic receptors and resulting in the classic signs of organophosphate poisoning (e.g., salivation, lacrimation, urination, defecation).

Question 152: A radiograph of the wrist in a 25-year-old athlete reveals a fracture of the proximal pole of the scaphoid. Which vascular anatomy principle makes this fracture MOST prone to avascular necrosis?

Avascular necrosis risk is highest in the waist, not the proximal pole
The scaphoid's blood supply enters distally and travels retrograde to the proximal pole (Correct answer)
The distal pole receives its blood supply last, making the proximal pole susceptible
The scaphoid has dual arterial supply from both the radial and ulnar arteries equally

Correct answer: The scaphoid's blood supply enters distally and travels retrograde to the proximal pole

The scaphoid receives its primary blood supply through branches of the radial artery that enter the bone at its distal dorsal ridge and travel retrograde (distally to proximally) through intraosseous vessels. A proximal pole fracture thus interrupts this retrograde flow, leaving the proximal fragment with no remaining vascular supply, resulting in a high rate of avascular necrosis. Waist fractures also carry risk but not as severely as proximal pole fractures.

Question 153: A 6-year-old male dog presents with recurrent urinary tract infections, struvite crystalluria, and radiodense bladder stones. What is the most likely underlying predisposing condition?

Calcium oxalate nephrolithiasis
Urease-producing bacterial infection (Correct answer)
Primary hyperaldosteronism
Hypercalcemia

Correct answer: Urease-producing bacterial infection

Urease-producing bacteria (e.g., Staphylococcus, Proteus) hydrolyze urea to ammonia, raising urine pH and promoting struvite crystal formation.

Question 154: Which hormone, produced by the posterior pituitary gland, promotes water reabsorption in the renal collecting ducts?

Aldosterone
Antidiuretic hormone (ADH) (Correct answer)
Parathyroid hormone
Atrial natriuretic peptide

Correct answer: Antidiuretic hormone (ADH)

Antidiuretic hormone (ADH, vasopressin) acts on aquaporin channels in collecting duct cells to increase water reabsorption and concentrate urine.

Question 155: A 35-year-old woman has elevated serum ALT, AST, and anti-smooth muscle antibody titer of 1:320. Liver biopsy shows interface hepatitis with plasma cell infiltration. What is the diagnosis?

Primary sclerosing cholangitis
Viral hepatitis C
Autoimmune hepatitis (Correct answer)
Primary biliary cholangitis

Correct answer: Autoimmune hepatitis

Autoimmune hepatitis features interface hepatitis, plasma cell-rich inflammation, and positive autoantibodies including anti-smooth muscle antibody.

Question 156: Serial progesterone values in a bitch are: Day 0 = 1.2 ng/mL, Day 2 = 4.9 ng/mL, Day 4 = 9.6 ng/mL. Vaginal cytology on Day 4 shows 95% superficial cells. Frozen-thawed semen intrauterine insemination is planned. What is the OPTIMAL timing for insemination?

Day 2 — the LH surge occurred as progesterone rose above 2 ng/mL, and frozen semen's short lifespan requires insemination at the earliest fertile window
Day 8–9 — ovulation does not occur until progesterone exceeds 15–20 ng/mL, so earlier insemination would precede the fertile window
Day 6 — approximately 4 days post-estimated LH surge, allowing oocyte maturation to metaphase II before insemination with short-lived frozen-thawed semen (Correct answer)
Day 4 immediately — 95% superficial cell cornification indicates peak fertility and frozen semen should be used without delay

Correct answer: Day 6 — approximately 4 days post-estimated LH surge, allowing oocyte maturation to metaphase II before insemination with short-lived frozen-thawed semen

The LH surge in dogs corresponds to the initial progesterone rise above ~1.5–2.0 ng/mL — estimated here around Day 1–2. Ovulation follows the LH surge by approximately 48 hours (~Day 3–4). Critically, canine oocytes are ovulated in primary oocyte stage (meiosis I arrest) and require an additional 48–72 hours of oviductal maturation to reach metaphase II (fertilizable stage). The earliest fertilizable window is therefore approximately Day 5–6 (4 days post-LH). Frozen-thawed semen has a viable lifespan of only 12–24 hours post-thaw, requiring precise timing at peak oocyte fertility. Day 6 optimally coincides oocyte maturation with viable sperm.

Question 157: During intestinal anastomosis, a surgeon performs a leak test. Which method is most commonly used intraoperatively to check for anastomotic leakage?

Occlude the bowel on both sides of the anastomosis and gently milk intraluminal air toward the site while submerging in saline (Correct answer)
Inject methylene blue through a nasogastric tube
Apply fluorescein dye under UV light
Request intraoperative endoscopy

Correct answer: Occlude the bowel on both sides of the anastomosis and gently milk intraluminal air toward the site while submerging in saline

The air-leak test involves occluding the bowel on each side, injecting air or milking intraluminal gas toward the anastomosis submerged in saline, and watching for bubbles indicating a leak.

Question 158: Which class of drugs causes hyperkalemia by blocking aldosterone's action in the collecting duct?

Thiazide diuretics (hydrochlorothiazide)
Loop diuretics (furosemide)
Osmotic diuretics (mannitol)
Potassium-sparing diuretics (spironolactone) (Correct answer)

Correct answer: Potassium-sparing diuretics (spironolactone)

Spironolactone competitively antagonizes aldosterone receptors in the collecting duct, preventing sodium reabsorption and potassium secretion, leading to potassium retention and hyperkalemia.

Question 159: A 72-year-old man with a longstanding monoclonal gammopathy undergoes kidney biopsy for worsening proteinuria and renal insufficiency. Congo red stain is negative. Light microscopy shows mesangial expansion with PAS-positive, non-fibrillary deposits. Electron microscopy reveals granular (non-fibrillary) electron-dense deposits in the mesangium and along the inner aspect of glomerular basement membranes. Immunofluorescence shows strong staining for IgG (kappa light chain restricted) but negative for lambda. What is the most likely diagnosis?

Monoclonal immunoglobulin deposition disease (MIDD), specifically light chain deposition disease (LCDD) (Correct answer)
Fibrillary glomerulonephritis with kappa restriction
Membranoproliferative glomerulonephritis type I secondary to cryoglobulinemia
AL amyloidosis with unusual ultrastructural appearance

Correct answer: Monoclonal immunoglobulin deposition disease (MIDD), specifically light chain deposition disease (LCDD)

MIDD (light chain or heavy chain deposition disease) presents with Congo red-negative, non-fibrillary granular deposits on EM — distinguishing it from amyloidosis (fibrils, Congo red positive) and fibrillary GN (larger, randomly arranged fibrils, ~20nm). Kappa light chain restriction is characteristic of LCDD (kappa >> lambda in LCDD). The subendothelial/mesangial granular deposits with mesangial expansion and IF showing monotypic kappa are hallmarks. MPGN/cryoglobulinemia would show organized deposits and typically polyclonal or different IF patterns.

Question 160: A patient develops masseter muscle rigidity immediately after succinylcholine administration. What is the most concerning diagnosis?

Malignant hyperthermia susceptibility (Correct answer)
Pseudocholinesterase deficiency
Normal fasciculation response
Laryngospasm

Correct answer: Malignant hyperthermia susceptibility

Masseter spasm after succinylcholine is a warning sign of malignant hyperthermia susceptibility and warrants discontinuation of triggering agents.

Question 161: The cardiotoxicity of digoxin overdose is primarily managed with:

Calcium gluconate infusion
Digoxin-specific antibody fragments (Digibind) (Correct answer)
Activated charcoal and dialysis
High-dose atropine boluses

Correct answer: Digoxin-specific antibody fragments (Digibind)

Digoxin-specific Fab antibody fragments bind free digoxin in the plasma, rapidly reversing toxicity; because digoxin has a large volume of distribution, dialysis is ineffective.

Question 162: A patient with a known dibucaine number of 20 receives succinylcholine for rapid sequence intubation. What is the most appropriate immediate management after failed spontaneous reversal at 45 minutes?

Give neostigmine 0.07 mg/kg with glycopyrrolate to antagonize the phase II block
Maintain sedation, continue controlled ventilation, and await plasma cholinesterase reconstitution (Correct answer)
Administer sugammadex 16 mg/kg to reverse the block
Infuse fresh frozen plasma to restore pseudocholinesterase activity rapidly

Correct answer: Maintain sedation, continue controlled ventilation, and await plasma cholinesterase reconstitution

A dibucaine number of 20 indicates homozygous atypical pseudocholinesterase — the patient cannot hydrolyze succinylcholine, resulting in prolonged (hours) paralysis. The correct management is to maintain sedation and mechanically ventilate until the block spontaneously dissipates. Sugammadex only encapsulates aminosteroid NDMBs (rocuronium/vecuronium) — it has no effect on succinylcholine. Neostigmine inhibits acetylcholinesterase and may worsen a phase II block rather than reliably reverse it. FFP contains pseudocholinesterase but response is inconsistent and takes time; it is not first-line.

Question 163: Avian species lack a diaphragm, yet achieve effective ventilation through a unique air sac system. In a bird at rest, which air sacs receive FRESH (oxygen-rich, CO2-poor) air during INSPIRATION?

Caudal thoracic and abdominal air sacs (Correct answer)
Cranial thoracic and cervical air sacs
All air sacs simultaneously, with parabronchi filtering the air bidirectionally
Only the interclavicular air sac, which distributes to the others via pressure gradients

Correct answer: Caudal thoracic and abdominal air sacs

Avian respiratory flow is unidirectional through the parabronchi across two full breathing cycles. During INSPIRATION, fresh air bypasses the lungs almost entirely and flows directly into the CAUDAL air sacs (caudal thoracic and abdominal). Simultaneously, air that was stored in the caudal sacs during the previous cycle moves through the parabronchi for gas exchange. During expiration, the now-spent air moves into the cranial air sacs and is expelled. This counter-current/cross-current system means that only the caudal sacs receive fresh inspired air — cranial sacs receive parabronchial effluent.

Question 164: A 6-year-old Golden Retriever is presented with lethargy, polyuria, and polydipsia. A complete blood count (CBC) and serum chemistry panel are performed. The CBC is unremarkable. The chemistry panel reveals a blood glucose of 450 mg/dL (normal: 75-120 mg/dL) and elevated liver enzymes. A urinalysis shows 4+ glucosuria and 2+ ketonuria. Which of the following is the most likely pathologic process responsible for these findings?

Glycogen storage disease in hepatocytes
Neoplastic proliferation of adrenal cortical cells
Renal tubular insensitivity to antidiuretic hormone
Immune-mediated destruction of pancreatic beta cells (Correct answer)

Correct answer: Immune-mediated destruction of pancreatic beta cells

The clinical signs (polyuria, polydipsia) combined with severe hyperglycemia, glucosuria, and ketonuria are classic for diabetes mellitus. In dogs, the most common cause of diabetes mellitus is the immune-mediated destruction of insulin-producing beta cells in the pancreatic islets, leading to absolute insulin deficiency.

Question 165: A lesion of the medial longitudinal fasciculus (MLF) produces which classic finding?

Homonymous hemianopia
Ipsilateral gaze palsy
Contralateral ptosis
Internuclear ophthalmoplegia (Correct answer)

Correct answer: Internuclear ophthalmoplegia

An MLF lesion disrupts communication between CN VI and contralateral CN III nuclei, causing internuclear ophthalmoplegia with impaired adduction ipsilaterally and nystagmus contralaterally.

Question 166: The femoral triangle contains all of the following EXCEPT:

Femoral artery
Obturator nerve (Correct answer)
Femoral nerve
Femoral vein

Correct answer: Obturator nerve

The femoral triangle contains the femoral nerve, artery, and vein (lateral to medial: nerve, artery, vein); the obturator nerve exits through the obturator foramen medially.

Question 167: A 4-year-old intact male ferret presents with bilateral symmetric alopecia, muscle wasting, and a palpable abdominal mass. Testosterone levels are markedly elevated. Testicular histology shows large, vacuolated Sertoli cells with disorganized tubular architecture and intratubular calcifications. What is the most likely diagnosis and the pathophysiologic mechanism of the alopecia?

Testicular torsion with secondary ischemic necrosis; stress-induced hypercortisolism causing alopecia
Sertoli cell tumor; hyperestrogenism from estrogen secretion by neoplastic Sertoli cells causing follicular arrest (Correct answer)
Seminoma; paraneoplastic immune-mediated follicular destruction unrelated to hormonal imbalance
Interstitial (Leydig) cell tumor; excess androgen production causing telogen effluvium via androgen receptor desensitization

Correct answer: Sertoli cell tumor; hyperestrogenism from estrogen secretion by neoplastic Sertoli cells causing follicular arrest

Sertoli cell tumors in ferrets (and dogs) frequently produce estrogen, leading to hyperestrogenism. Despite elevated testosterone (which may be co-secreted), the neoplastic Sertoli cells' estrogen production causes bilateral symmetric alopecia by inducing follicular arrest in telogen. Bone marrow suppression (aplastic anemia, thrombocytopenia) is another life-threatening sequela. Vacuolated Sertoli cells with disorganized tubules and calcifications are hallmarks of this tumor type.

Question 168: A veterinarian submits a feline nasal biopsy to the laboratory. Histopathology reveals broad-based budding yeasts with narrow-neck attachments between parent and daughter cells, and organisms ranging from 5–15 µm with thick, refractile walls. GMS stain highlights the organisms. Culture on SDA at 25°C produces a mold; at 37°C, the same isolate produces yeast colonies. This thermal dimorphism and the histologic morphology are MOST consistent with which organism, and what is its primary ecological niche?

Blastomyces dermatitidis; moist acidic sandy soil near waterways in endemic regions (Correct answer)
Sporothrix schenckii; decaying vegetation and soil, with infection via traumatic inoculation
Histoplasma capsulatum; soil enriched with bat or avian guano in river valleys
Paracoccidioides brasiliensis; soil in Latin American coffee-growing regions, primarily affecting dogs

Correct answer: Blastomyces dermatitidis; moist acidic sandy soil near waterways in endemic regions

The combination of broad-based budding (as opposed to the narrow-necked multiple buds of Paracoccidioides), large thick-walled yeast forms (5–15 µm), thermal dimorphism, and feline nasal involvement is characteristic of Blastomyces dermatitidis. Blastomycosis is endemic in the Ohio and Mississippi River valleys and the Great Lakes region, associated with moist, acidic, sandy or loamy soils near waterways. The 'broad-based bud' is the pathognomonic histologic feature. Histoplasma is smaller (2–4 µm, intracellular) and has narrow-necked budding; Sporothrix has cigar-shaped yeast forms and is rarely isolated from nasal biopsy in cats with systemic disease.

Question 169: A 72-year-old woman with chronic kidney disease (GFR 18 mL/min) is prescribed metformin by an urgent care provider. Three days later she presents obtunded with an arterial pH of 7.08, lactate of 14 mmol/L, and normal glucose. What is the pathophysiologic mechanism of her toxicity?

Metformin activates AMPK in renal tubular cells, suppressing ATP synthesis and causing systemic lactic acidosis
Accumulated metformin inhibits mitochondrial complex I, impairing oxidative phosphorylation and forcing anaerobic lactate production (Correct answer)
Metformin undergoes renal bioactivation to a toxic quinone metabolite that alkylates hepatocyte mitochondria
Metformin competitively inhibits gluconeogenesis from pyruvate, causing pyruvate to accumulate and convert to lactate

Correct answer: Accumulated metformin inhibits mitochondrial complex I, impairing oxidative phosphorylation and forcing anaerobic lactate production

Metformin-associated lactic acidosis (MALA) occurs primarily through inhibition of mitochondrial complex I (NADH dehydrogenase) of the electron transport chain. This impairs oxidative phosphorylation, forces cells into anaerobic glycolysis, and generates excess lactate. In renal impairment, metformin accumulates to toxic concentrations because it is cleared almost entirely by glomerular filtration. Metformin does inhibit hepatic gluconeogenesis (its therapeutic mechanism), but MALA's root cause is mitochondrial respiratory chain dysfunction.

Question 170: A public health department implements a new screening program for colorectal cancer using fecal immunochemical testing (FIT). Among 10,000 screened individuals, 200 test positive; of these, 50 actually have colorectal cancer. Among those who test negative, 10 actually have colorectal cancer. Which statement best describes the program's positive predictive value (PPV) and its primary determinant in this population?

PPV = 83%; primarily determined by the high specificity of the test
PPV = 5%; primarily determined by the high false-negative rate
PPV = 25%; primarily determined by the low sensitivity of FIT in this population
PPV = 25%; primarily determined by the low prevalence of colorectal cancer in the screened population (Correct answer)

Correct answer: PPV = 25%; primarily determined by the low prevalence of colorectal cancer in the screened population

PPV = True Positives / All Positives = 50/200 = 25%. The total disease cases = 50 + 10 = 60 out of 10,000, giving a prevalence of 0.6%. In low-prevalence populations, even a highly specific test yields a low PPV because the base rate of disease is so small that false positives outnumber true positives. This is the primary determinant — disease prevalence — not sensitivity alone.

Question 171: Which of the following wound closure materials would be CONTRAINDICATED in a contaminated or infected wound?

Polydioxanone (PDS)
Braided multifilament suture such as polyglactin 910 placed in the wound bed (Correct answer)
Poliglecaprone 25 (monofilament absorbable)
Nylon monofilament

Correct answer: Braided multifilament suture such as polyglactin 910 placed in the wound bed

Braided multifilament sutures have capillary action that harbors bacteria, making them contraindicated in infected or heavily contaminated wound beds.

Question 172: Which bacterial pathogen is the most common cause of leptospirosis in dogs in the United States?

Ehrlichia canis
Leptospira interrogans serovars Canicola and Icterohaemorrhagiae (Correct answer)
Brucella canis
Borrelia burgdorferi

Correct answer: Leptospira interrogans serovars Canicola and Icterohaemorrhagiae

Leptospira interrogans serovars Canicola and Icterohaemorrhagiae are the serovars most frequently associated with leptospirosis in US dogs.

Question 173: Which pharmacokinetic model best describes the distribution of intravenous anesthetics after a bolus dose?

Three-compartment model (Correct answer)
Michaelis-Menten kinetics
One-compartment model
Zero-order kinetics

Correct answer: Three-compartment model

IV anesthetics are best described by a three-compartment model: central (plasma), rapidly equilibrating peripheral (vessel-rich tissues), and slowly equilibrating peripheral (muscle, fat).

Question 174: A 4-year-old thoroughbred racehorse presents with exercise intolerance and a grade 4/6 holosystolic murmur loudest at the left cardiac apex. What is the most likely diagnosis?

Tricuspid regurgitation
Mitral regurgitation (Correct answer)
Ventricular septal defect
Aortic regurgitation

Correct answer: Mitral regurgitation

Mitral regurgitation produces a holosystolic murmur heard loudest at the left cardiac apex (mitral valve area) in horses.

Question 175: A 10-year-old mare in the third trimester of pregnancy presents with sudden onset of severe weakness, recumbency, and muscle trembling. Serum chemistry shows calcium 5.8 mg/dL. Which treatment should be administered immediately?

IM magnesium sulfate
Slow IV calcium borogluconate (Correct answer)
Oral calcium propionate gel
IV glucose (50% dextrose)

Correct answer: Slow IV calcium borogluconate

Hypocalcemia in late-gestation mares is treated with slow IV administration of calcium borogluconate while monitoring heart rate to avoid cardiac arrhythmias.

Question 176: On MRI of the brain, periventricular white matter T2/FLAIR hyperintensities perpendicular to the corpus callosum ('Dawson's fingers') are characteristic of which condition?

Watershed infarcts
Progressive multifocal leukoencephalopathy
Hypertensive small vessel disease
Multiple sclerosis (Correct answer)

Correct answer: Multiple sclerosis

Dawson's fingers are ovoid MS plaques oriented along perivenular drainage pathways perpendicular to the ventricles, a hallmark of MS on MRI.

Question 177: A swine producer is designing a comprehensive biosecurity plan to prevent the introduction of Porcine Reproductive and Respiratory Syndrome (PRRS) into their naive sow herd. Of the following options, which is considered the highest risk and therefore the most critical control point to manage?

Requiring all personnel and visitors to shower-in and shower-out.
Controlling rodent and insect populations within the barns.
Ensuring all feed is sourced from a single, reputable mill.
The introduction of new animals, particularly replacement gilts and semen. (Correct answer)

Correct answer: The introduction of new animals, particularly replacement gilts and semen.

The introduction of infected pigs (live animals) or contaminated semen is the most frequent and highest-risk route for PRRS virus to enter a previously negative herd. Therefore, strict protocols for sourcing, isolating, and testing all incoming animals and genetic material are the most critical components of a biosecurity plan aimed at preventing PRRS introduction.

Question 178: What is the name of the specialized myocardial cells that form the ventricular conduction pathway from the bundle of His to the ventricular walls?

Gap junctions
Bachmann's bundle fibers
Intercalated discs
Purkinje fibers (Correct answer)

Correct answer: Purkinje fibers

Purkinje fibers are large, rapidly conducting cardiac muscle cells that transmit the electrical impulse from the bundle branches to the ventricular myocardium.

Question 179: Which test is considered the 'gold standard' for diagnosis of Johne's disease (paratuberculosis) in cattle?

ELISA serology
Intradermal Johnin test
Fecal culture for Mycobacterium avium subsp. paratuberculosis (Correct answer)
Complete blood count

Correct answer: Fecal culture for Mycobacterium avium subsp. paratuberculosis

Fecal culture for Mycobacterium avium subsp. paratuberculosis remains the gold standard, though it requires 8–16 weeks of incubation.

Question 180: What is the primary purpose of using retractors during a surgical procedure?

To maintain exposure by holding tissues away from the operative field without continuous manual effort (Correct answer)
To cut and dissect through tissue planes
To measure wound tension before closure
To apply hemostasis to bleeding vessels

Correct answer: To maintain exposure by holding tissues away from the operative field without continuous manual effort

Retractors hold tissue out of the operative field, providing consistent exposure and freeing the surgeon's and assistant's hands for dissection and other tasks.

Question 181: A researcher finds that a novel drug has a volume of distribution (Vd) of 450 L in a 70 kg patient and a plasma protein binding of 95%. Which of the following conclusions is most pharmacokinetically accurate?

High protein binding accounts for the large Vd because albumin carries the drug into tissue compartments
A Vd of 450 L indicates the drug undergoes significant first-pass hepatic extraction before reaching systemic circulation
The drug extensively distributes into peripheral tissues, likely via lipophilicity or tissue binding, and plasma concentration reflects only a small fraction of total body drug (Correct answer)
The large Vd implies rapid renal clearance because unbound drug at tissues is efficiently filtered at the glomerulus

Correct answer: The drug extensively distributes into peripheral tissues, likely via lipophilicity or tissue binding, and plasma concentration reflects only a small fraction of total body drug

Vd of 450 L far exceeds total body water (~42 L) and plasma volume (~3 L), indicating extensive sequestration into peripheral tissues — typically driven by lipophilicity, intracellular binding, or high tissue affinity. High plasma protein binding (95%) actually works against tissue distribution by retaining drug in plasma; the large Vd despite high binding means the drug's affinity for tissue sites overwhelms its affinity for plasma proteins. Vd is a mathematical construct and does not directly inform about hepatic extraction or renal clearance kinetics.

Question 182: A 7-year-old Doberman Pinscher with a history of von Willebrand disease (type I) is scheduled for elective gastropexy. Preoperative vWF antigen level is 18% (reference >60%). Which perioperative hemostatic management strategy is MOST appropriate prior to making the incision?

Pretreat with cryoprecipitate 1 unit per 10 kg body weight given 4 hours before surgery to allow equilibration
Transfuse fresh frozen plasma (FFP) at 10 mL/kg immediately before incision to replace vWF directly
Administer epsilon-aminocaproic acid (EACA) IV loading dose prior to surgery to inhibit fibrinolysis
Administer desmopressin (DDAVP) 1 µg/kg IV 30 minutes before incision to stimulate endogenous vWF release from endothelial storage granules (Correct answer)

Correct answer: Administer desmopressin (DDAVP) 1 µg/kg IV 30 minutes before incision to stimulate endogenous vWF release from endothelial storage granules

In type I von Willebrand disease, endothelial cells retain functional Weibel-Palade bodies containing vWF. Desmopressin (DDAVP) stimulates V2 receptors on endothelial cells, triggering release of stored vWF multimers and acutely raising vWF levels — often 2–3× baseline — within 30–60 minutes. This is the preferred first-line approach for elective procedures in type I vWD. Cryoprecipitate is the most vWF-concentrated blood product and is indicated when DDAVP response is insufficient or for types II/III vWD; however, its administration 4 hours before is suboptimal (it should be given just before incision due to short vWF half-life). FFP contains vWF but at lower concentration and requires large volumes. EACA addresses fibrinolysis, not the primary platelet adhesion defect.

Question 183: Microscopically, an atherosclerotic plaque contains foam cells. These foam cells are derived from which cell type?

Endothelial cells
Macrophages (Correct answer)
Neutrophils
Smooth muscle cells

Correct answer: Macrophages

Foam cells in atherosclerotic plaques are lipid-laden macrophages that have engulfed oxidized LDL via scavenger receptors.

Question 184: Which electrocardiographic finding is most consistent with hyperkalemia in a dog presenting with weakness and bradycardia?

Right bundle branch block pattern
Prolonged QT interval
ST segment depression
Tall, peaked T waves and absent P waves (Correct answer)

Correct answer: Tall, peaked T waves and absent P waves

Hyperkalemia causes tall peaked T waves, widened QRS, and eventually loss of P waves as atrial standstill develops.

Question 185: Which organism is the primary cause of contagious equine metritis (CEM) in mares?

Klebsiella pneumoniae (capsule type 1)
Taylorella equigenitalis (Correct answer)
Streptococcus equi subsp. zooepidemicus
Pseudomonas aeruginosa

Correct answer: Taylorella equigenitalis

Taylorella equigenitalis is a fastidious Gram-negative coccobacillus that causes CEM, a reportable venereal disease of horses in the US.

Question 186: On an abdominal X-ray, 'thumbprinting' of the colon wall is most associated with which condition?

Diverticulosis
Ischemic colitis (Correct answer)
Colon polyps
Irritable bowel syndrome

Correct answer: Ischemic colitis

Thumbprinting represents submucosal edema and hemorrhage causing nodular indentations of the bowel wall, classic for ischemic colitis.

Question 187: Which combination of Five Domains model criteria best captures the welfare state of a laboratory beagle undergoing repeated aversive conditioning trials but showing no overt pain behaviors and maintaining normal bodyweight?

Impaired in Domain 2 (Environment) and Domain 5 (Mental State) primarily
Impaired only in Domain 5 (Mental State) because all physical domains are intact
Impaired in Domain 3 (Health) and Domain 4 (Behavioral Interactions) only
Impaired in Domain 4 (Behavioral Interactions) and Domain 5 (Mental State) primarily (Correct answer)

Correct answer: Impaired in Domain 4 (Behavioral Interactions) and Domain 5 (Mental State) primarily

The Five Domains framework (Mellor et al.) recognizes that welfare comprises physical domains (Nutrition, Environment, Health, Behavioral Interactions) that feed into Domain 5 (Mental State). Repeated aversive conditioning primarily compromises Domain 4 (Behavioral Interactions—the animal's capacity to express normal species-typical behavior and control social/environmental interactions) and Domain 5 (Mental State—negative affective experience, fear, helplessness). Physical domains may be intact, but the framework explicitly requires assessment of functional behavior and affective state, not just physical health markers.

Question 188: Which method is considered the gold standard for early pregnancy diagnosis in cattle?

Milk progesterone at day 18
Transrectal ultrasonography at day 25–28 (Correct answer)
Rectal palpation at day 25
Serum progesterone at day 21

Correct answer: Transrectal ultrasonography at day 25–28

Transrectal ultrasonography can reliably detect a bovine embryonic vesicle as early as day 25–28 with high accuracy and allows assessment of embryo viability.

Question 189: A 3-year-old child presents with hematuria, a flank mass, and hypertension. Nephrectomy reveals a tumor with blastemal, stromal, and epithelial components. What is the diagnosis?

Neuroblastoma
Rhabdomyosarcoma
Wilms tumor (nephroblastoma) (Correct answer)
Renal cell carcinoma

Correct answer: Wilms tumor (nephroblastoma)

Wilms tumor (nephroblastoma) is the most common renal malignancy in children and characteristically shows triphasic histology with blastemal, stromal, and epithelial elements.

Question 190: A 3-week-old calf presents with profuse watery diarrhea, severe dehydration, and weak suckle reflex. Blood gas shows pH 7.08, bicarbonate 8 mEq/L, and strong ion gap elevation. Which pathogen is most consistent with this clinical picture in calves of this age?

Escherichia coli K99 (F5)
Bovine rotavirus
Salmonella typhimurium
Cryptosporidium parvum (Correct answer)

Correct answer: Cryptosporidium parvum

Cryptosporidium parvum is the predominant cause of profuse, watery scours in calves aged 5–21 days with severe metabolic acidosis due to D-lactic acid production.

Question 191: A cat presents with a ventromedial corneal opacity that appears yellow-brown and is firmly adherent to the cornea. There is no fluorescein stain uptake. What is the most likely diagnosis?

Eosinophilic keratitis
Corneal sequestrum (corneal nigrum) (Correct answer)
Anterior lens luxation
Indolent corneal ulcer

Correct answer: Corneal sequestrum (corneal nigrum)

Corneal sequestrum (also called corneal nigrum or corneal mummification) is unique to cats and is characterized by a focal area of corneal stromal necrosis that appears amber, brown, or black. It does not stain with fluorescein (no epithelial defect initially) and is firmly embedded in the stroma. It is most common in brachycephalic breeds. Treatment is surgical keratectomy. Eosinophilic keratitis has a white/pink vascularized plaque appearance; indolent ulcers have loose epithelial edges and do stain.

Question 192: Bovine viral diarrhea virus (BVDV) infection during which gestational window produces persistently infected (PI) calves?

Last trimester (days 180–280)
At parturition
First 30 days of gestation
Days 40–125 of gestation (Correct answer)

Correct answer: Days 40–125 of gestation

PI calves result from fetal infection between approximately days 40 and 125, before immune competence develops, causing immunologic tolerance to the virus.

Question 193: In the context of humane endpoints, which criterion should trigger early euthanasia in a rodent tumor model?

Tumor volume exceeding 10% of body weight or causing mobility impairment (Correct answer)
Tumor detection by imaging only
Loss of fur color around the tumor
Any palpable tumor mass regardless of size

Correct answer: Tumor volume exceeding 10% of body weight or causing mobility impairment

Humane endpoints specify objective criteria—typically tumor burden relative to body mass or functional impairment—to limit suffering before the scientific endpoint.

Question 194: Which of the following electrolyte abnormalities is most commonly associated with grain overload (carbohydrate engorgement) in cattle?

Hypernatremia
Hyperkalemia
Metabolic acidosis with D-lactatemia (Correct answer)
Hypocalcemia

Correct answer: Metabolic acidosis with D-lactatemia

Grain overload causes rapid fermentation producing D-lactic acid, leading to metabolic acidosis and D-lactatemia, which can be severe and life-threatening.

Question 195: In the male dog, the bulbus glandis engorges during copulation to create a 'tie' with the female. Which erectile tissue type composes the bulbus glandis, and how does it differ from the corpus cavernosum penis in terms of venous drainage control?

The bulbus glandis is composed of corpus spongiosum tissue and lacks a tunica albuginea, relying on ischiocavernosus muscle compression rather than trabecular veno-occlusion for engorgement
The bulbus glandis is composed of corpus spongiosum tissue with a prominent tunica albuginea; venous drainage is occluded by contraction of the bulbospongiosus muscle compressing emissary veins (Correct answer)
The bulbus glandis is composed of corpus cavernosum tissue with a thick tunica albuginea that traps blood via veno-occlusion, identical to the mechanism in the corpus cavernosum penis
The bulbus glandis is a distinct erectile structure composed of sinusoidal erectile tissue not homologous to either corpus cavernosum or corpus spongiosum, maintained by arteriovenous shunt closure

Correct answer: The bulbus glandis is composed of corpus spongiosum tissue with a prominent tunica albuginea; venous drainage is occluded by contraction of the bulbospongiosus muscle compressing emissary veins

The bulbus glandis is an expansion of the corpus spongiosum (pars longa glandis surrounds the urethra, while the bulbus glandis flanks it proximally). Unlike the corpus cavernosum penis (which has a rigid tunica albuginea enabling high-pressure erection), the bulbus glandis has its own tunica and depends primarily on bulbospongiosus muscle activity compressing emissary veins to maintain engorgement. This sustained muscular compression is what makes the copulatory tie functionally possible for 15–30 minutes.

Question 196: What is the recommended dietary protein level (% dry matter) for healthy adult maintenance in dogs according to AAFCO guidelines?

10% dry matter minimum
30% dry matter minimum
18% dry matter minimum (Correct answer)
25% dry matter minimum

Correct answer: 18% dry matter minimum

AAFCO sets the minimum dietary protein at 18% dry matter for adult dog maintenance, with 22.5% required for growth and reproduction.

Question 197: A 45-year-old woman presents with progressive dysphagia and a mediastinal mass on imaging. Biopsy reveals cells with abundant pale cytoplasm, prominent nucleoli, and a 'chicken-wire' vascular pattern. Immunohistochemistry shows positivity for CD138 and negativity for CD45. Which translocation is most characteristically associated with this neoplasm?

t(8;14)(q24;q32)
t(11;14)(q13;q32) (Correct answer)
t(15;17)(q22;q21)
t(14;18)(q32;q21)

Correct answer: t(11;14)(q13;q32)

The histologic and immunohistochemical features describe a plasmacytoma/plasma cell myeloma. The t(11;14)(q13;q32) translocation, which juxtaposes CCND1 (cyclin D1) to the IGH locus, is the hallmark translocation in plasma cell neoplasms (multiple myeloma variant) and also mantle cell lymphoma. The 'chicken-wire' vascularity and CD138+/CD45- profile confirm plasma cell lineage. t(14;18) is characteristic of follicular lymphoma; t(8;14) of Burkitt lymphoma; t(15;17) of acute promyelocytic leukemia.

Question 198: A dairy herd is observed to have significantly elevated cortisol levels and reduced milk yield despite adequate nutrition and housing space. Behavioral sampling reveals that subordinate cows spend 40% less time at the feed bunk than dominant individuals. Which welfare intervention is MOST likely to address the root cause?

Administering exogenous oxytocin to improve milk letdown
Separating the herd into dominant and subordinate subgroups permanently
Increasing the feed bunk space-to-cow ratio to at least 0.6 meters per animal (Correct answer)
Increasing the photoperiod to 16 hours of light to stimulate appetite

Correct answer: Increasing the feed bunk space-to-cow ratio to at least 0.6 meters per animal

The behavioral evidence points to competitive exclusion at the feed bunk as the welfare issue — subordinate cows are being displaced, resulting in restricted feed intake and the associated stress response. The gold-standard intervention is increasing feed bunk linear space to ≥0.6 m/cow, which reduces agonistic interactions and allows simultaneous access. Permanent herd separation disrupts established social hierarchies and creates repeated regrouping stress. Oxytocin addresses the symptom (reduced letdown) not the cause. Extended photoperiod affects DMI but does not resolve the competition-based access problem.

Question 199: In cattle with bovine respiratory disease (BRD), which pathogen is most commonly implicated as the primary viral initiator that predisposes animals to secondary bacterial pneumonia?

Infectious bovine rhinotracheitis virus (IBR/BHV-1)
Bovine viral diarrhea virus (BVDV) (Correct answer)
Bovine respiratory syncytial virus (BRSV)
Bovine parainfluenza-3 virus (PI-3)

Correct answer: Bovine viral diarrhea virus (BVDV)

BVDV causes immunosuppression by infecting and depleting lymphocytes, dramatically increasing susceptibility to secondary bacterial pathogens in the BRD complex.

Question 200: An autopsy on a 55-year-old male with a history of chronic hypertension shows concentric left ventricular wall thickening with a normal or reduced chamber size. What is this change called?

Dilated cardiomyopathy
Eccentric hypertrophy
Hypertrophic obstructive cardiomyopathy
Concentric hypertrophy (Correct answer)

Correct answer: Concentric hypertrophy

Concentric hypertrophy results from pressure overload (as in hypertension), causing sarcomeres to add in parallel, increasing wall thickness while preserving or reducing chamber size.

Question 201: A biopsy from a skin mass on an 8-year-old Boxer is submitted for histopathology. The pathologist describes 'sheets of round cells with distinct cell borders, abundant eosinophilic granular cytoplasm, and central, round nuclei.' There is minimal pleomorphism and a low mitotic rate. Mast cell granules are not observed with routine H&E staining but would likely be prominent with which special stain?

Prussian Blue
Masson's Trichrome
Toluidine Blue (Correct answer)
Periodic acid-Schiff (PAS)

Correct answer: Toluidine Blue

The description is highly suggestive of a mast cell tumor, which is common in dogs, particularly Boxer breeds. The characteristic cytoplasmic granules of mast cells are metachromatic, meaning they stain a different color than the dye itself. Toluidine blue is a classic metachromatic stain used to highlight mast cell granules, which will stain purple against a blue background.

Question 202: A horse in Florida develops crusting dermatitis with intensely pruritic skin lesions in summer. Skin scraping reveals no mites, but the lesions improve when the horse is stabled at night. The most likely causative agent is:

Chorioptes equi
Habronema microstoma larvae
Culicoides spp. hypersensitivity (Correct answer)
Psoroptes equi

Correct answer: Culicoides spp. hypersensitivity

Sweet itch (Queensland itch, summer seasonal recurrent dermatitis) is caused by IgE-mediated hypersensitivity to the saliva of Culicoides biting midges, which are most active at dawn and dusk. Stabling at night reduces exposure and improves signs. Chorioptes and Psoroptes are mange mites identified on scraping. Habronema causes cutaneous habronemiasis (summer sores) with granulomatous lesions, not diffuse pruritic crusting.

Question 203: Which oncogene is characteristically amplified in neuroblastoma and correlates with poor prognosis?

HER2/NEU
BCR-ABL
RET
N-MYC (Correct answer)

Correct answer: N-MYC

N-MYC (MYCN) amplification in neuroblastoma is associated with rapid tumor progression and poor prognosis.

Question 204: Neospora caninum abortion storms occur on a cattle farm. The farmer asks whether culling all seropositive cows will eliminate the infection from the herd. Which statement best explains the limitation of this strategy?

N. caninum tachyzoites can survive for months in soil, reinfecting seronegative cattle through pasture contact
The cattle serological test cross-reacts with Toxoplasma gondii antibodies, making accurate identification of true positives impossible
N. caninum forms resistant oocysts in cattle intestinal epithelium, enabling direct cattle-to-cattle horizontal fecal-oral transmission
Vertical (transplacental) transmission perpetuates infection in offspring of seropositive dams, and some persistently infected calves remain seronegative early in life (Correct answer)

Correct answer: Vertical (transplacental) transmission perpetuates infection in offspring of seropositive dams, and some persistently infected calves remain seronegative early in life

The primary challenge in Neospora caninum control is highly efficient vertical (transplacental) transmission — congenitally infected calves born to seropositive dams are themselves infected and will eventually seroconvert. Additionally, congenitally infected calves may be seronegative in the first weeks of life before maternal antibody wanes and their own response develops. Culling seropositive adults removes the most obvious reservoir but misses already-infected offspring. N. caninum does not complete its sexual cycle (oocyst production) in cattle — only in canids — so cattle-to-cattle fecal-oral transmission does not occur. Serology reliably distinguishes N. caninum from T. gondii.

Question 205: During a cadaveric dissection of the hand, a student notes that a small muscle originates from the hook of the hamate and the flexor retinaculum, and inserts into the medial border of the fifth metacarpal. Paralysis of this muscle would most impair which movement?

Adduction of the little finger
Abduction of the little finger (Correct answer)
Flexion of the little finger at the MCP joint
Opposition of the little finger

Correct answer: Abduction of the little finger

The muscle described — originating from the hook of the hamate and flexor retinaculum and inserting into the medial border of the fifth metacarpal — is the abductor digiti minimi. It is the prime mover for little finger abduction at the MCP joint. The opponens digiti minimi (also from the hamate) inserts along the shaft of the fifth metacarpal to produce opposition. Flexor digiti minimi brevis flexes the proximal phalanx. Adduction is produced by the palmar interossei.

Question 206: A 4-year-old child ingests an unknown quantity of a grandparent's iron supplement tablets. In the emergency department, the child develops metabolic acidosis, coagulopathy, and elevated serum ferritin. Which mechanism accounts for the metabolic acidosis seen in acute iron toxicity?

Iron inhibits carbonic anhydrase in red blood cells, impairing CO2 transport
Iron-induced emesis causes chloride loss, generating a hypochloremic alkalosis that is masked by concurrent lactic acidosis
Free ferric iron catalyzes the Fenton reaction generating hydroxyl radicals that disrupt mitochondrial oxidative phosphorylation, causing lactic acidosis (Correct answer)
Iron chelates bicarbonate in the bloodstream, directly consuming buffer

Correct answer: Free ferric iron catalyzes the Fenton reaction generating hydroxyl radicals that disrupt mitochondrial oxidative phosphorylation, causing lactic acidosis

In acute iron toxicity, excess free iron (Fe²⁺/Fe³⁺) catalyzes the Fenton reaction (Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + ·OH), generating highly reactive hydroxyl radicals. These radicals cause lipid peroxidation of mitochondrial membranes, uncoupling oxidative phosphorylation and shifting cellular metabolism to anaerobic glycolysis. The resultant lactate accumulation produces an elevated anion gap metabolic acidosis. This is compounded by direct cellular toxicity to hepatocytes and GI epithelium.

Question 207: Aspergillus fumigatus nasal granulomas in dogs are best diagnosed by which combination?

Rhinoscopy with visualization of plaques plus fungal culture or cytology confirming Aspergillus (Correct answer)
Complete blood count showing eosinophilia
Serum ELISA alone
Thoracic radiographs

Correct answer: Rhinoscopy with visualization of plaques plus fungal culture or cytology confirming Aspergillus

Definitive diagnosis of nasal aspergillosis requires rhinoscopic visualization of characteristic white fungal plaques combined with cytology or culture confirmation.

Question 208: Which pharmacokinetic property most accurately explains why context-sensitive half-time increases dramatically for fentanyl but NOT for remifentanil during prolonged infusions?

Fentanyl undergoes extensive hepatic first-pass elimination while remifentanil does not
Remifentanil is hydrolyzed by nonspecific plasma and tissue esterases independent of organ blood flow (Correct answer)
Fentanyl's high lipid solubility traps it in the CNS, prolonging effect-site concentration
Remifentanil has a smaller volume of distribution at steady state

Correct answer: Remifentanil is hydrolyzed by nonspecific plasma and tissue esterases independent of organ blood flow

Remifentanil contains a methyl ester linkage that is cleaved by ubiquitous nonspecific esterases in blood and tissues, giving it a context-insensitive offset (half-time ~3–5 min regardless of infusion duration). Fentanyl, despite redistribution early on, accumulates in fat and muscle peripheral compartments during prolonged infusion; when the infusion stops, drug continues to transfer back to plasma, markedly extending clinical effect. While remifentanil does have a smaller Vdss, this alone does not fully account for its unique kinetics — the ester metabolism is the defining mechanism.

Question 209: A county health department implements a universal lead screening program for all children aged 1–2 years. After one year, the prevalence of elevated blood lead levels (≥5 µg/dL) appears to have increased compared to pre-program data. Which of the following best explains this phenomenon?

The case definition threshold was lowered, reducing specificity of the test
The screening program identified previously undetected cases, inflating apparent prevalence (Correct answer)
Healthy worker effect biased the pre-program baseline downward
Lead exposure in the county has genuinely worsened due to environmental contamination

Correct answer: The screening program identified previously undetected cases, inflating apparent prevalence

This is a classic example of 'surveillance artifact' or 'ascertainment bias.' Before universal screening, only symptomatic or high-risk children were tested, so many true cases went undetected. The denominator (all children screened) is now larger, but so is the numerator (newly identified cases). The apparent rise in prevalence reflects improved case-finding, not a true increase in disease burden. This phenomenon is commonly seen when new or expanded screening programs are introduced.

Question 210: During exploratory celiotomy in a 4-year-old Labrador, you encounter a mesenteric mass with encasement of a segmental branch of the cranial mesenteric artery. Histopathology later confirms mesenteric lymphoma. Intraoperatively, which vascular principle BEST guides your decision regarding resection margins and bowel viability?

The antimesenteric border is the last region to lose perfusion after mesenteric vessel ligation and can be preserved selectively
Mesenteric arterial arcades provide redundant anastomotic flow, so single arcade ligation rarely requires bowel resection (Correct answer)
Venous drainage of the jejunum is independent of arterial supply patterns, allowing venous ligation without arterial compromise
The small intestine receives segmental, non-collateralizing end-arterial branches from arcades; ligation of an arcade disrupts all bowel perfusion distal to that point along its distribution

Correct answer: Mesenteric arterial arcades provide redundant anastomotic flow, so single arcade ligation rarely requires bowel resection

The jejunum and ileum have a rich series of anastomosing arterial arcades (primary, secondary, and in some species tertiary arcades) originating from the cranial mesenteric artery. These arcades interconnect and provide collateral circulation, meaning ligation of a single arcade branch does not necessarily devascularize the bowel it supplies — adjacent arcades can compensate. However, ligation of multiple sequential arcades or the primary trunk eliminates this redundancy and mandates resection. Understanding this arcade anatomy is critical when planning mass excision near mesenteric vessels.

Question 211: The anatomical position of the appendix is most commonly:

Subcecal
Retrocecal (Correct answer)
Pelvic (descending into true pelvis)
Paracolic

Correct answer: Retrocecal

In approximately 65% of individuals, the appendix lies in a retrocecal position, which can make the presentation of appendicitis atypical.

Question 212: Sugammadex reverses neuromuscular blockade by which mechanism?

Competing with aminosteroid NMBAs at nicotinic receptors
Encapsulating rocuronium or vecuronium molecules (Correct answer)
Enhancing acetylcholine release from motor nerve terminals
Inhibiting acetylcholinesterase at the neuromuscular junction

Correct answer: Encapsulating rocuronium or vecuronium molecules

Sugammadex is a modified gamma-cyclodextrin that encapsulates and inactivates steroidal neuromuscular blocking agents like rocuronium and vecuronium.

Question 213: A cat is diagnosed with a diaphragmatic hernia following blunt trauma. Which organ is most commonly herniated into the thorax in cats?

Liver and small intestine (Correct answer)
Spleen
Kidney
Stomach

Correct answer: Liver and small intestine

In cats with traumatic diaphragmatic hernia, the liver and small intestine are the most frequently herniated organs due to their proximity to the diaphragm.

Question 214: A 4-year-old male intact Bernese Mountain Dog presents with progressive ascites, hepatomegaly, and elevated liver enzymes. Abdominal ultrasound reveals a severely distended caudal vena cava and hepatic veins with absent pulsatility on Doppler, and a markedly thickened hepatic vein ostium. Portosystemic shunting is absent. Protein-losing nephropathy and protein-losing enteropathy have been ruled out. Which diagnostic test should be prioritized next?

Measurement of serum alpha-1-proteinase inhibitor fecal loss to rule out PLE as an occult cause of hypoalbuminemia driving the ascites.
Right heart catheterization or cardiac CT angiography to evaluate for constrictive pericarditis or Budd-Chiari-like syndrome from hepatic vein or caval obstruction. (Correct answer)
Liver biopsy via ultrasound-guided Tru-Cut needle to rule out hepatic veno-occlusive disease (sinusoidal obstruction syndrome) from pyrrolizidine alkaloid toxicity.
CT-guided fine-needle aspirate of the thickened hepatic vein ostium region to rule out intravascular lymphoma as the cause of mechanical obstruction.

Correct answer: Right heart catheterization or cardiac CT angiography to evaluate for constrictive pericarditis or Budd-Chiari-like syndrome from hepatic vein or caval obstruction.

The clinical presentation — ascites, hepatomegaly, distended caudal vena cava with hepatic venous outflow obstruction and absent pulsatility on Doppler — is characteristic of Budd-Chiari syndrome or post-hepatic (post-sinusoidal) portal hypertension. In dogs, this can result from right-sided heart failure, constrictive pericarditis, caval thrombosis, or extrinsic compression. Right heart catheterization or cardiac CT angiography is the priority to distinguish cardiac causes (e.g., constrictive pericarditis, which is surgically correctable) from primary vascular obstruction. Liver biopsy before excluding post-hepatic causes risks bleeding and misses the diagnosis; sinusoidal obstruction syndrome does not produce dilated hepatic veins on Doppler.

Question 215: A 62-year-old male with known prostate cancer undergoes a bone scan showing multiple areas of increased radionuclide uptake throughout the axial skeleton. However, a repeat scan 6 weeks after initiating hormonal therapy reveals dramatically REDUCED uptake in all lesions. Which of the following BEST explains this scintigraphic finding?

Cold spots representing photopenic metastases replacing normal bone
Complete resolution of osseous metastatic disease
Superscan pattern indicating diffuse marrow replacement
'Flare phenomenon' indicating a paradoxical favorable treatment response (Correct answer)

Correct answer: 'Flare phenomenon' indicating a paradoxical favorable treatment response

The 'flare phenomenon' is a well-recognized pitfall in oncologic bone scintigraphy: effective treatment of osteoblastic metastases triggers an acute healing response with increased osteoblastic activity, paradoxically causing transiently INCREASED uptake on the next scan (or subsequently, as lesions heal and remodel, dramatically DECREASED uptake as reparative sclerosis stabilizes). Reduced uptake after therapy in previously active lesions reflects sclerotic healing (mineralizing woven bone replacing vascular tumor), not tumor disappearance — correlation with morphologic imaging (CT/MRI) is required. True complete resolution would need biopsy confirmation; a superscan shows diffusely elevated uptake; cold spots indicate photopenic lesions.

Question 216: Borna disease virus causes encephalitis primarily in horses in central Europe. To which order does this neurotropic negative-sense RNA virus belong?

Nidovirales
Picornavirales
Herpesvirales
Mononegavirales (Correct answer)

Correct answer: Mononegavirales

Borna disease virus belongs to the family Bornaviridae within the order Mononegavirales, which includes other non-segmented negative-sense RNA viruses like rhabdoviruses and paramyxoviruses.

Question 217: A heavily pregnant cow is found in lateral recumbency and unable to rise. She is alert but unable to stand despite calcium treatment. Her selenium and vitamin E levels are low. What condition should be considered?

White muscle disease (nutritional myodegeneration) (Correct answer)
Grass tetany (hypomagnesemia)
Downer cow syndrome with sciatic nerve damage
Transit tetany (transport hypocalcemia)

Correct answer: White muscle disease (nutritional myodegeneration)

White muscle disease from selenium/vitamin E deficiency causes myodegeneration that may present as recumbency in adult cattle; treatment with selenium and vitamin E supplementation is indicated.

Question 218: A 40-year-old with history of ulcerative colitis undergoes colectomy. The specimen shows dysplastic crypts with complex back-to-back glands and nuclear stratification confined to the mucosa. What is the most appropriate diagnosis?

Reactive atypia
Adenocarcinoma
Low-grade dysplasia
High-grade dysplasia (Correct answer)

Correct answer: High-grade dysplasia

High-grade dysplasia in inflammatory bowel disease is characterized by complex architectural distortion and marked nuclear atypia without invasion through the muscularis mucosae.

Question 219: Which of the following correctly describes the pharmacodynamic concept of 'insurmountable antagonism' and distinguishes it from competitive antagonism?

Insurmountable antagonists shift the agonist dose-response curve to the right with no change in maximum efficacy, whereas competitive antagonists depress the maximum response even at high agonist concentrations
Insurmountable antagonists act at allosteric sites only, while competitive antagonists exclusively occupy the orthosteric binding site
Insurmountable antagonists bind covalently or with extremely high affinity to the receptor, depressing the maximum agonist response regardless of agonist concentration, whereas competitive antagonists shift the curve rightward while preserving Emax (Correct answer)
Insurmountable antagonists have a higher affinity for the receptor than agonists but can be displaced by sufficiently high agonist concentrations, whereas competitive antagonists cannot be displaced

Correct answer: Insurmountable antagonists bind covalently or with extremely high affinity to the receptor, depressing the maximum agonist response regardless of agonist concentration, whereas competitive antagonists shift the curve rightward while preserving Emax

Insurmountable (non-competitive or pseudo-irreversible) antagonism occurs when an antagonist binds with such high affinity (effectively irreversibly, e.g., covalently) that increasing agonist concentration cannot restore the full maximum response—Emax is depressed. This contrasts with surmountable (competitive) antagonism, where the antagonist is reversible and an agonist at sufficiently high concentrations can outcompete it, restoring Emax while shifting the EC50 rightward (parallel shift). Note that an insurmountable antagonist doesn't need to act allosterically—some orthosteric antagonists are insurmountable if their binding is effectively irreversible.

Question 220: A 6-year-old Suffolk ewe presents with progressive weight loss, submandibular edema, and pasture-mate deaths over 2 months. Fecal egg count: 12,000 EPG Haemonchus contortus. You initiate treatment with monepantel (amino-acetonitrile derivative). Three weeks later, repeat FEC shows 9,800 EPG. A fecal egg count reduction test (FECRT) confirms <50% reduction. ACR-8 gene sequencing of recovered larvae reveals a premature stop codon. What is the mechanism of resistance demonstrated?

Single nucleotide polymorphism in the latrophilin receptor lat-1 disrupting drug-receptor binding affinity
Gain-of-function in P-glycoprotein efflux pump encoded by pgp-9, increasing drug expulsion
Cytochrome P450 CYP-35A upregulation causing accelerated monepantel hydroxylation and inactivation
Loss-of-function mutation in the DEG-3 subfamily acetylcholine receptor subunit ACR-8, eliminating the drug target (Correct answer)

Correct answer: Loss-of-function mutation in the DEG-3 subfamily acetylcholine receptor subunit ACR-8, eliminating the drug target

Monepantel acts on nematode-specific DEG-3 subfamily acetylcholine receptors, particularly the ACR-8 subunit. Resistance to monepantel is mediated by loss-of-function mutations (including premature stop codons, frameshift mutations) in acr-8, which eliminate the target receptor entirely. This is a target-site resistance mechanism distinct from the efflux-pump (pgp-9) mechanism seen with macrocyclic lactones or metabolic resistance. Latrophilin receptors are targeted by emodepside, not monepantel.

Question 221: Which of the following suture patterns provides the greatest resistance to leakage in a low-tension enterotomy closure in small animals, while minimizing luminal narrowing compared to a single-layer interrupted appositional pattern?

Connell pattern followed by a Cushing pattern inverting layer
Simple continuous appositional (e.g., Cushing-Cushing double layer inverting)
Single-layer simple interrupted appositional using 3-0 polydioxanone
Gambee pattern using a full-thickness monofilament absorbable suture (Correct answer)

Correct answer: Gambee pattern using a full-thickness monofilament absorbable suture

The Gambee pattern (full-thickness interrupted suture that incorporates mucosa, submucosa, and muscularis but everts slightly to appose mucosa-to-mucosa) provides excellent leak resistance comparable to double-layer inverting techniques, with significantly less luminal narrowing. Double-layer inverting patterns (Cushing over Connell) invert considerable tissue and reduce lumen diameter. Simple interrupted appositional is acceptable but the Gambee pattern is specifically validated for superior burst strength and minimal stenosis in intestinal surgery.

Question 222: When performing a lateral thoracotomy in a dog, between which intercostal spaces is the incision most commonly made to access the caudal lung lobes and esophagus?

2nd–3rd intercostal space
8th–9th intercostal space (Correct answer)
11th–12th intercostal space
4th–5th intercostal space

Correct answer: 8th–9th intercostal space

The 8th–9th intercostal space provides optimal access to the caudal lung lobes, caudal esophagus, and caudal mediastinal structures in dogs.

Question 223: Which structure passes through the foramen ovale of the skull base?

CN V2 (maxillary nerve)
CN VI (abducens nerve)
CN V3 (mandibular nerve) (Correct answer)
Middle meningeal artery

Correct answer: CN V3 (mandibular nerve)

The mandibular nerve (CN V3) passes through the foramen ovale to exit the middle cranial fossa.

Question 224: Which level of prevention does administering HPV vaccine to adolescents represent?

Primordial prevention
Tertiary prevention
Primary prevention (Correct answer)
Secondary prevention

Correct answer: Primary prevention

HPV vaccination prevents initial infection and subsequent cancer, making it primary prevention by reducing disease incidence.

Question 225: Which tick-borne protozoan infects erythrocytes of cattle and causes a hemolytic anemia known as Texas cattle fever (redwater fever) in the southern United States?

Babesia bovis (Correct answer)
Cytauxzoon felis
Anaplasma marginale
Theileria parva

Correct answer: Babesia bovis

Babesia bovis, transmitted by Rhipicephalus (Boophilus) microplus, infects bovine erythrocytes causing intravascular hemolysis, hemoglobinuria, and high mortality.

Basic and Clinical Sciences Examination (BCSE)

The BCSE is administered by the AVMA ECFVG program to assess veterinary graduates' knowledge of basic and clinical sciences at the level expected of an entry-level US veterinarian. It consists of 225 multiple-choice questions completed in 220 minutes.

225

Questions

220 min

Time Limit

70%

To Pass

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