Electrolyte Disorders USMLE Step 1 Practice Question
A 75-year-old woman with heart failure on furosemide and ibuprofen presents with acute confusion. Vital signs show BP 92/58, HR 108, RR 20. Labs reveal Na+ 124 mEq/L, serum osmolality 265 mOsm/kg, urine osmolality 580 mOsm/kg, urine Na+ 45 mEq/L, and BUN 32 mg/dL with normal creatinine. Physical exam confirms hypovolemia with dry mucous membranes. Urine output is reduced. Which mechanism best explains her hyponatremia?
Answer choices
- AAdrenal insufficiency from chronic loop diuretic use
- BPrimary SIADH from CNS disease or malignancy unmasked by diuretics
- CNSAID-induced acute kidney injury with uremic hyponatremia
- DDilutional hyponatremia from excessive free water intake
- EDiuretic-induced volume depletion stimulating ADH release despite low serum osmolalityCorrect answer
- FThiazide diuretic-induced nephrogenic diabetes insipidus causing free water loss
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