A 58-year-old man with small cell lung carcinoma develops hyponatremia with urine osmolality higher than serum osmolality. Which paraneoplastic mechanism is responsible?
- A Ectopic ACTH secretion causing adrenal hyperstimulation
- B PTH-rP secretion causing hypercalcemia-mediated aquaporin upregulation
- C Ectopic ADH (vasopressin) secretion causing SIADH ✓
- D Lambert-Eaton antibodies blocking renal tubular calcium channels
Correct answer: C. Ectopic ADH (vasopressin) secretion causing SIADH
Explanation
Small cell lung carcinoma frequently secretes ectopic antidiuretic hormone (ADH/vasopressin), producing SIADH: hypotonic hyponatremia with inappropriately concentrated urine (urine osmolality > serum osmolality). Ectopic ACTH from SCLC causes Cushing syndrome, not hyponatremia. PTH-rP causes hypercalcemia, a distinct paraneoplastic syndrome seen more in squamous cell carcinoma. Lambert-Eaton syndrome involves neuromuscular junction dysfunction, not renal water handling.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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