A 70-year-old male with a history of heart failure and hypertension is admitted with acute worsening of confusion. His labs show Na 120 mEq/L, K 4.0 mEq/L, Cr 1.2 mg/dL. Urine osmolality is 450 mOsm/kg and urine sodium is 35 mEq/L. He is euvolemic on exam. What is the most likely diagnosis for his hyponatremia?
✓ Correct Answer: Option A
Euvolemic hyponatremia with inappropriately high urine osmolality (>100 mOsm/kg) and elevated urine sodium (>20 mEq/L) strongly suggests SIADH. Heart failure causes hypervolemic hyponatremia.