A paraneoplastic syndrome of hyponatremia, concentrated urine, and normal adrenal function is found in a patient with small-cell lung carcinoma. The ectopic product responsible for this syndrome is:

• A Ectopic ACTH
• B Ectopic PTHrP
• C Ectopic erythropoietin
• D Ectopic ADH (SIADH) ✓

Explanation

Small-cell lung carcinoma most commonly secretes ectopic ADH (antidiuretic hormone), producing SIADH with dilutional hyponatremia, inappropriately concentrated urine, and normal renal and adrenal function. Ectopic ACTH causes Cushing syndrome with hypokalemia. PTHrP from squamous cell carcinoma causes hypercalcemia. Erythropoietin from renal cell carcinoma causes polycythemia.

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

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