Calcium and Parathyroid USMLE Step 1 Practice Question
A 45-year-old man with end-stage renal disease on hemodialysis presents with progressive bone pain and muscle weakness. Vital signs: BP 148/92, HR 88, RR 16, Temp 37°C, SpO2 98%. Serum calcium 10.5 mg/dL, phosphate 7.2 mg/dL, PTH 450 pg/mL. Parathyroid scintigraphy reveals bilaterally enlarged glands. Despite calcium carbonate supplementation, PTH remains elevated. No signs of acute infection. Which mechanism best explains the persistent secondary hyperparathyroidism?
Answer choices
- ALoss of normal negative feedback due to renal phosphate retentionCorrect answer
- BSuppressed 1,25-dihydroxyvitamin D synthesis
- CHypomagnesemia preventing PTH suppression
- DDevelopment of parathyroid carcinoma
- EVitamin D deficiency causing primary hyperparathyroidism
- FTertiary hyperparathyroidism with autonomous PTH secretion independent of calcium-phosphate regulation
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations — plus adaptive practice that targets your weak areas — with a free MedBoardPRO account.