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SIADH — USMLE Step 3 MCQ

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HardInternal Medicine (chronic management)SIADHUSMLE Step 3

A 70-year-old man with small cell lung cancer has confusion and sodium 121 mEq/L. Serum osmolality is low, urine osmolality is high, and urine sodium is elevated. He is euvolemic on examination and has normal thyroid and adrenal testing. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: B — Inappropriate antidiuretic hormone secretion

SIADH causes euvolemic hypotonic hyponatremia with inappropriately concentrated urine and elevated urine sodium.

Reference: European Hyponatremia Guideline