Calcium and Parathyroid USMLE Step 1 Practice Question
A 28-year-old woman with newly diagnosed systemic sarcoidosis presents with fatigue and polyuria. Vital signs: BP 118/76, HR 88, RR 16, Temp 37.2°C, SpO₂ 98%. Laboratory studies reveal serum calcium 12.1 mg/dL, 24-hour urine calcium 420 mg/24h, PTH 8 pg/mL (normal 15-65), and 1,25-dihydroxyvitamin D 156 pg/mL (normal 19-67). Chest radiograph shows hilar lymphadenopathy. Which mechanism best explains her hypercalcemia?
Answer choices
- AImpaired renal excretion of calcium due to interstitial nephritis
- BExtrarenal conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D by activated macrophages in granulomasCorrect answer
- CPTH-mediated increased renal tubular reabsorption of calcium
- DPTHrP secretion from sarcoid granulomas stimulating bone resorption
- EDirect osteoclast activation by TNF-alpha produced by T cells
- FIncreased intestinal absorption of calcium due to elevated PTH-independent expression of calcium-binding proteins
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