Electrolyte Disorders USMLE Step 1 Practice Question
A 72-year-old man with a 10-year history of COPD presents to the emergency department with acute confusion, headache, and nausea that developed over 24 hours. His wife reports he has been more lethargic than usual and complained of a mild cough 1 week ago that has since resolved. He takes furosemide 40 mg daily for volume management. On examination, vital signs are: BP 128/76 mmHg, HR 88/min, RR 18/min, temperature 37°C. Physical examination reveals no edema, normal skin turgor, and no focal neurologic deficits. Laboratory studies show: Serum sodium: 118 mEq/L (reference 135-145)
Serum osmolality: 245 mOsm/kg (reference 280-295)
Urine osmolality: 680 mOsm/kg
Urine sodium: 95 mEq/L
Creatinine: 0.9 mg/dL
TSH: 2.1 mIU/L (normal)
Chest X-ray: Mild chronic changes, no acute infiltrate Which of the following is the most likely explanation for this patient's hyponatremia?
Answer choices
- AHypovolemic hyponatremia from diuretic-induced volume depletion
- BPrimary polydipsia with excessive free water intake
- CSyndrome of inappropriate antidiuretic hormone secretion from paraneoplastic processCorrect answer
- DCentral diabetes insipidus with impaired free water excretion
- EHypothyroidism with decreased renal free water clearance
- FNephrotic syndrome with secondary hyperaldosteronism and fluid retention
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