Calcium and Parathyroid USMLE Step 1 Practice Question
A 51-year-old man with chronic kidney disease stage 4 (GFR 18 mL/min) presents with bone pain and muscle cramps. Vital signs: BP 152/88, HR 92, RR 18, Temp 37°C, SpO2 98%. Laboratory studies reveal serum calcium 7.8 mg/dL, phosphate 7.2 mg/dL, and PTH 285 pg/mL. Alkaline phosphatase is normal. He denies taking phosphate binders or vitamin D supplements. Which pathophysiologic mechanism initiates secondary hyperparathyroidism in this patient's CKD stage 4?
Answer choices
- AMagnesium excess from dialysate
- BExcessive dietary calcium intake causing PTH stimulation
- CDecreased renal 1-alpha hydroxylase activity and impaired 1,25-vitamin D productionCorrect answer
- DPrimary parathyroid adenoma development
- EDirect PTH suppression by hyperphosphatemia
- FFGF-23 as the earliest CKD-MBD signal rising before phosphate and PTH and directly inhibiting 1-alpha-hydroxylase
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