Calcium and Parathyroid USMLE Step 1 Practice Question
A 35-year-old man with stage 4 chronic kidney disease (GFR 18 mL/min/1.73m²) presents with bone pain and muscle cramps. Vital signs: BP 148/92, HR 88, RR 16, Temp 37°C. Labs show serum calcium 7.8 mg/dL, phosphate 6.2 mg/dL, PTH 620 pg/mL, and 25-hydroxyvitamin D 15 ng/mL. Intact parathyroids visualized on ultrasound. He denies recent fractures. Which pathophysiologic mechanism best explains his markedly elevated PTH?
Answer choices
- AIncreased sensitivity of parathyroid glands to FGF23 stimulation
- BPrimary defect in parathyroid hormone synthesis genes
- CPTH-secreting adenoma unrelated to renal disease
- DAutonomous parathyroid gland hyperfunction independent of calcium levels
- EDecreased phosphate excretion leading to hyperphosphatemia and secondary hyperparathyroidismCorrect answer
- FImpaired conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D due to loss of renal 1α-hydroxylase activity
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