Calcium and Parathyroid USMLE Step 1 Practice Question
A 72-year-old woman on chronic hydrochlorothiazide for hypertension presents with asymptomatic hypercalcemia. Vital signs: BP 142/88 mmHg, HR 76/min, RR 16/min, Temp 37.2°C. Laboratory findings: serum calcium 11.5 mg/dL, phosphate 3.5 mg/dL, PTH 18 pg/mL, 25-OH vitamin D 38 ng/mL, and 24-hour urine calcium 180 mg/24h. Serum creatinine is normal. Parathyroid imaging shows no adenoma. Which mechanism best explains these findings?
Answer choices
- AThiazide-induced hyperparathyroidism
- BSecondary hyperparathyroidism from sodium depletion
- CThiazide-induced suppression of PTH secretion
- DVitamin D toxicity from increased intestinal absorption
- EUnmasking of underlying primary hyperparathyroidism by thiazideCorrect answer
- FThiazide-induced increase in parathyroid hormone-related peptide (PTHrP) secretion from bone
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