Calcium and Parathyroid USMLE Step 1 Practice Question
A 72-year-old man with severe COPD exacerbation presents with dyspnea, hypoxemia (SpO2 88%), and tachycardia (HR 112). Laboratory studies reveal serum calcium 4.2 mg/dL, phosphate 6.8 mg/dL, and PTH 180 pg/mL. Notably, serum magnesium is 1.1 mg/dL (normal 1.7–2.2 mg/dL). He denies recent diuretic use. Chest X-ray confirms acute infiltrates without infiltrative disease. Which mechanism best explains his resistant hypocalcemia unresponsive to PTH?
Answer choices
- ACalcitonin excess from lung malignancy
- BAcute phosphate poisoning
- CPrimary hypoparathyroidism
- DHypomagnesemia impairing PTH secretion and actionCorrect answer
- EVitamin D deficiency alone
- FAcute respiratory acidosis causing precipitation of calcium phosphate in tissues
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