Electrolyte Disorders USMLE Step 1 Practice Question
A 62-year-old man with stage 5 chronic kidney disease on hemodialysis presents with muscle cramps and pruritus. Vital signs: BP 148/92 mmHg, HR 88/min, RR 16/min, temp 37°C. Laboratory studies show phosphate 6.8 mg/dL, calcium 7.6 mg/dL, elevated PTH at 680 pg/mL, and normal 25-hydroxyvitamin D levels. Urinalysis shows no proteinuria. Which of the following best explains his hypocalcemia?
Answer choices
- AReduced intestinal calcium absorption due to decreased calcitriol productionCorrect answer
- BIncreased urinary calcium excretion due to hyperaldosteronism
- CIncreased bone uptake from vitamin A intoxication
- DEnhanced distal tubule calcium reabsorption due to thiazide use
- EExcess calcitonin secretion from thyroid C cells
- FDecreased serum albumin leading to reduced total calcium levels without affecting ionized calcium
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