Electrolyte Disorders USMLE Step 1 Practice Question
A 52-year-old man with a 10-year history of systolic heart failure (EF 35%) presents to the emergency department with progressive orthostatic dizziness, confusion, and nausea over the past 3 days. His current medications include furosemide 80 mg daily, lisinopril 20 mg daily, and spironolactone 25 mg daily. Vital signs: supine BP 98/62, standing BP 88/54; HR 102; RR 16; temp 37°C; SpO2 98% on room air. Physical examination reveals 2+ pitting edema of the lower extremities, elevated jugular venous pressure at 8 cm H2O, and clear lung bases. Laboratory studies show: serum sodium 128 mEq/L (normal 135-145), serum osmolality 260 mOsm/kg, BUN 28 mg/dL, creatinine 1.4 mg/dL (baseline 1.0). Urine studies show: osmolality 580 mOsm/L, sodium 45 mEq/L, specific gravity 1.018. Which of the following best explains this patient's clinical presentation and laboratory abnormalities?
Answer choices
- AHypovolemic hyponatremia secondary to excessive diuretic use and poor oral intake
- BEuvolemic hyponatremia from SIADH due to diuretic-induced volume depletion
- CHypervolemic hyponatremia secondary to heart failure with impaired free water excretionCorrect answer
- DPrimary hypothyroidism with myxedema
- ECentral adrenal insufficiency from chronic ACE inhibitor use
- FNephrogenic diabetes insipidus complicating chronic diuretic therapy
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