A 58-year-old man with small cell lung carcinoma develops hyponatremia, decreased serum osmolality, and concentrated urine. Which paraneoplastic mechanism best explains this finding?

• A Ectopic ACTH secretion causing mineralocorticoid excess
• B PTHrP secretion causing hypercalcemia-induced natriuresis
• C ANP secretion from tumor cells causing natriuresis
• D Ectopic ADH (vasopressin) secretion causing SIADH ✓

Explanation

Small cell lung carcinoma is the classic tumor associated with SIADH due to ectopic ADH secretion, resulting in euvolemic hyponatremia with concentrated urine. Ectopic ACTH causes Cushing syndrome, not hyponatremia. PTHrP causes hypercalcemia (humoral hypercalcemia of malignancy). ANP secretion is not a recognized paraneoplastic syndrome.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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