MD Infectious Disease and Microbiology 4 — Questions and Answers
Question 1: A 30-year-old with sickle cell disease presents with fever and severe bone pain. Blood culture grows encapsulated gram-negative rods. Which organism is this patient uniquely susceptible to due to functional asplenia?
- Staphylococcus aureus
- Streptococcus pneumoniae
- Salmonella species (Correct answer)
- Pseudomonas aeruginosa
Correct answer: Salmonella species
Sickle cell patients with functional asplenia are uniquely susceptible to Salmonella osteomyelitis (and S. pneumoniae sepsis), because the spleen normally opsonizes these encapsulated organisms.
Question 2: A 22-year-old develops fever, severe headache, petechial rash, and nuchal rigidity. CSF shows >1000 WBCs (predominantly neutrophils), low glucose, and gram-negative diplococci. Which antibiotic regimen is most appropriate?
- Vancomycin + metronidazole
- Ceftriaxone + dexamethasone (Correct answer)
- Ampicillin + gentamicin
- Meropenem alone
Correct answer: Ceftriaxone + dexamethasone
Neisseria meningitidis meningitis is treated with ceftriaxone; adjunctive dexamethasone reduces inflammation and is recommended before or with the first antibiotic dose to prevent complications.
Question 3: A 55-year-old pigeon handler develops progressive pneumonia with relative bradycardia (pulse-temperature dissociation), dry cough, and hyponatremia. CXR shows patchy infiltrates. Which organism causes this atypical pneumonia?
- Mycoplasma pneumoniae
- Chlamydophila psittaci
- Legionella pneumophila (Correct answer)
- Coxiella burnetii
Correct answer: Legionella pneumophila
Legionella pneumophila causes Legionnaires' disease, characterized by relative bradycardia, hyponatremia, diarrhea, and pontiac fever-like prodrome; it grows in water-cooling systems and causes lobar pneumonia.
Question 4: A 45-year-old with AIDS (CD4 60 cells/μL) develops progressive bilateral visual loss described as 'pizza pie' fundus with perivascular hemorrhages and exudates. Which organism is responsible?
- Toxoplasma gondii
- Pneumocystis jirovecii
- Cytomegalovirus (Correct answer)
- Herpes simplex virus-1
Correct answer: Cytomegalovirus
CMV retinitis is the most common ocular opportunistic infection in AIDS patients with CD4 <50, producing the classic 'pizza pie' or 'scrambled eggs' appearance with hemorrhagic necrosis on funduscopy.
Question 5: A 6-year-old develops red strawberry tongue, scarlatiniform rash, and perioral pallor. Throat swab grows beta-hemolytic group A streptococcus. Which complication can be prevented by treating with penicillin?
- Post-streptococcal glomerulonephritis
- Rheumatic fever (Correct answer)
- Toxic shock syndrome
- Peritonsillar abscess
Correct answer: Rheumatic fever
Antibiotic treatment of group A streptococcal pharyngitis prevents acute rheumatic fever (but NOT post-streptococcal glomerulonephritis, which is not prevented by antibiotics).
Question 6: A patient with a prosthetic heart valve develops fever and a new murmur 2 months after valve replacement. Blood cultures grow coagulase-negative staphylococci. Which species is most likely?
- Staphylococcus epidermidis (Correct answer)
- Staphylococcus saprophyticus
- Staphylococcus lugdunensis
- Staphylococcus haemolyticus
Correct answer: Staphylococcus epidermidis
Staphylococcus epidermidis is the most common cause of early prosthetic valve endocarditis (within 1 year), as it colonizes implanted devices via biofilm formation.
Question 7: A 35-year-old woman develops fever, dysuria, flank pain, and costovertebral angle tenderness. Urine culture grows >100,000 CFU/mL of a gram-negative rod that is oxidase-negative and ferments lactose. Which organism?
- Pseudomonas aeruginosa
- Klebsiella pneumoniae
- Escherichia coli (Correct answer)
- Proteus mirabilis
Correct answer: Escherichia coli
E. coli is the most common cause of uncomplicated pyelonephritis, fermenting lactose (pink colonies on MacConkey agar) and being oxidase-negative, with the P fimbriae mediating renal tropism.
MD Infectious Disease and Microbiology 4
A 30-year-old with sickle cell disease presents with fever and severe bone pain.
Blood culture grows encapsulated gram-negative rods.
Which organism is this patient uniquely susceptible to due to functional asplenia?