The paraneoplastic syndrome most characteristically associated with small cell carcinoma of the lung is which condition?

• A. Cushing syndrome
• B. Hypercalcemia
• C. SIADH
• D. Lambert-Eaton myasthenic syndrome

Answer: (C)

Small cell lung carcinoma commonly secretes hormones from neuroendocrine cells, producing paraneoplastic syndromes. The most classic is SIADH, where ectopic ADH causes excessive water reabsorption, leading to hyponatremia with low serum osmolality and inappropriately concentrated urine, often in a euvolemic patient. While other paraneoplastic effects like ectopic ACTH can occur (Cushing syndrome) and hypercalcemia can arise in different cancers, and Lambert-Eaton may accompany small cell, SIADH is the hallmark association with this tumor type.