Calcium and Parathyroid USMLE Step 1 Practice Question
A 64-year-old man with stage 4 chronic kidney disease (GFR 18 mL/min/1.73m²) presents with fatigue and bone pain. Vital signs: BP 148/92 mmHg, HR 88/min, RR 16/min, temp 37°C. Labs show serum calcium 7.2 mg/dL, phosphate 6.1 mg/dL, PTH 412 pg/mL, and 1,25-vitamin D 14 pg/mL (normal >30). Serum albumin is normal. He denies polyuria. Which pathophysiologic mechanism primarily explains these laboratory abnormalities?
Answer choices
- AIntestinal malabsorption of calcium and vitamin D
- BParathyroid adenoma development independent of renal disease
- CDecreased extrarenal 1,25-vitamin D production by macrophages
- DReduced renal conversion of 25-OH vitamin D to 1,25-vitamin D and phosphate retentionCorrect answer
- EExcessive PTH suppression by elevated calcium levels
- FPrimary hyperparathyroidism with concurrent vitamin D deficiency from malnutrition
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