STP Clinical Sciences 2 — Questions and Answers
Question 1: A patient presents with elevated serum calcium, suppressed PTH, and elevated PTHrP. Which underlying condition is most likely?
- Primary hyperparathyroidism
- Humoral hypercalcemia of malignancy (Correct answer)
- Vitamin D toxicity
- Familial hypocalciuric hypercalcemia
Correct answer: Humoral hypercalcemia of malignancy
Elevated PTHrP with suppressed PTH and hypercalcemia is the hallmark of humoral hypercalcemia of malignancy, most commonly from squamous cell carcinomas.
Question 2: Which enzyme deficiency in the complement pathway results in recurrent Neisseria infections due to impaired membrane attack complex formation?
- C3 deficiency
- C1q deficiency
- Terminal complement (C5-C9) deficiency (Correct answer)
- Factor D deficiency
Correct answer: Terminal complement (C5-C9) deficiency
Deficiencies in terminal complement components (C5-C9) prevent formation of the membrane attack complex, specifically impairing defense against encapsulated Gram-negative organisms like Neisseria.
Question 3: A 45-year-old woman has a hemoglobin of 7.2 g/dL, MCV 110 fL, and elevated homocysteine with normal methylmalonic acid. Which deficiency is most likely?
- Vitamin B12 deficiency
- Folate deficiency (Correct answer)
- Iron deficiency
- Copper deficiency
Correct answer: Folate deficiency
Elevated homocysteine with normal methylmalonic acid indicates folate deficiency, since methylmalonic acid is elevated only in B12 deficiency, not folate deficiency.
Question 4: In flow cytometry-based diagnosis of paroxysmal nocturnal hemoglobinuria (PNH), which surface markers are characteristically absent?
- CD19 and CD20
- CD3 and CD4
- CD55 and CD59 (Correct answer)
- CD34 and CD117
Correct answer: CD55 and CD59
PNH is caused by a somatic PIG-A mutation leading to loss of GPI-anchored proteins CD55 (DAF) and CD59 (protectin), which normally protect red cells from complement lysis.
Question 5: A clinical scientist is validating a new diagnostic assay. The assay identifies 90 true positives and 10 false negatives from 100 confirmed disease cases. What is the sensitivity?
- 80%
- 85%
- 90% (Correct answer)
- 95%
Correct answer: 90%
Sensitivity = TP / (TP + FN) = 90 / (90 + 10) = 90%, representing the assay's ability to correctly identify true disease cases.
Question 6: Which type of renal tubular acidosis (RTA) is associated with hypokalemia and inability to acidify urine below pH 5.5?
- Type 1 (distal) RTA (Correct answer)
- Type 2 (proximal) RTA
- Type 4 RTA
- Type 3 RTA
Correct answer: Type 1 (distal) RTA
Type 1 (distal) RTA is caused by failure of alpha-intercalated cells to secrete H+ in the collecting duct, resulting in inability to acidify urine below pH 5.5 and associated hypokalemia.
Question 7: In Gram staining, which step differentiates Gram-positive from Gram-negative bacteria?
- Application of crystal violet primary stain
- Addition of iodine mordant
- Decolorization with alcohol/acetone (Correct answer)
- Counterstaining with safranin
Correct answer: Decolorization with alcohol/acetone
Decolorization with alcohol or acetone is the critical differentiating step; Gram-positive organisms retain the crystal violet-iodine complex due to their thick peptidoglycan wall, while Gram-negative organisms are decolorized.
STP Clinical Sciences 2
A patient presents with elevated serum calcium, suppressed PTH, and elevated PTHrP.
Which underlying condition is most likely?