Electrolyte Disorders USMLE Step 1 Practice Question
A 45-year-old man with chronic kidney disease stage 4 (GFR 22 mL/min/1.73m²) presents to the emergency department with acute-onset muscle cramps and generalized weakness. He denies recent medication changes, dietary indiscretions, or use of NSAIDs. Vital signs are BP 148/92 mmHg, HR 68 bpm, RR 16, and temperature 37°C. Physical examination reveals decreased muscle strength bilaterally. Laboratory studies show: serum potassium 6.8 mEq/L, serum calcium 8.9 mg/dL (normal 8.5-10.5), serum creatinine 3.2 mg/dL, and arterial blood gas shows pH 7.32 with HCO3− 18 mEq/L. A 12-lead ECG demonstrates peaked T waves, prolonged PR interval, and widened QRS complex. Which of the following is the most appropriate initial intervention?
Answer choices
- ASodium polystyrene sulfonate 15 g orally with sorbitol
- BCalcium gluconate 10% IV over 2-3 minutesCorrect answer
- CFurosemide 80 mg IV with aggressive normal saline hydration
- DInsulin 10 units IV with 25 mL of 50% dextrose
- ESodium bicarbonate 50 mEq IV followed by hemodialysis
- FSpironolactone 25 mg orally for potassium-sparing diuresis
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