A 58-year-old man with small cell lung carcinoma develops hyponatremia with urine osmolality > plasma osmolality. Which paraneoplastic mechanism is responsible?

• A Ectopic ACTH secretion causing aldosterone suppression
• B Ectopic PTHrP secretion causing renal sodium loss
• C Ectopic ADH (AVP) secretion causing SIADH ✓
• D Tumor-induced ANP release causing dilutional hyponatremia

Explanation

Small cell lung carcinoma is the classic cause of paraneoplastic SIADH due to ectopic secretion of arginine vasopressin (ADH). This leads to inappropriate water retention, dilutional hyponatremia, and concentrated urine despite low serum osmolality. PTHrP causes hypercalcemia, not hyponatremia, and ectopic ACTH would cause hypokalemic alkalosis.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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