A 58-year-old man with small cell lung carcinoma develops hyponatremia and concentrated urine despite normal renal function. Which paraneoplastic mechanism best explains this?
- A Ectopic ADH (AVP) secretion causing SIADH ✓
- B Ectopic ACTH secretion causing aldosterone excess
- C Ectopic PTHrP secretion causing hypercalcemia-induced nephrogenic DI
- D Tumor-induced destruction of posterior pituitary
Correct answer: A. Ectopic ADH (AVP) secretion causing SIADH
Explanation
Small cell lung carcinoma is the classic tumor associated with ectopic ADH (arginine vasopressin) secretion, producing SIADH. The result is euvolemic hyponatremia with a urine osmolality inappropriately higher than plasma osmolality. PTHrP causes hypercalcemia (hypercalciuria and nephrogenic DI), and ectopic ACTH produces Cushing syndrome, not hyponatremia.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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