Thyroid Disorders USMLE Step 1 Practice Question
A 63-year-old man with stage 4 chronic kidney disease presents with progressive diffuse bone pain and muscle weakness. Vital signs: BP 148/92 mmHg, HR 88/min, RR 16/min, temp 37.2°C. Laboratory studies reveal ionized calcium 6.8 mg/dL (low), phosphate 5.2 mg/dL (elevated), PTH 487 pg/mL (markedly elevated), and normal 25-hydroxyvitamin D levels. He denies recent fractures. Which pathophysiologic mechanism best explains his endocrine abnormality?
Answer choices
- AAutonomous PTH secretion by parathyroid adenoma
- BReduced phosphate excretion and decreased calcitriol productionCorrect answer
- CSuppressing antibodies against the TSH receptor
- DIncreased renal conversion of vitamin D to calcitriol
- EEnd organ resistance to parathyroid hormone
- FIncreased intestinal absorption of phosphate and enhanced osteoclastic bone resorption
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