A 65-year-old chronic smoker presents with confusion and nausea. Labs show serum sodium 118 mEq/L, serum osmolality 255 mOsm/kg, urine osmolality 450 mOsm/kg, and urine sodium 45 mEq/L. Chest X-ray shows a central hilar mass. Which histological type of lung carcinoma is most likely responsible?

• A Adenocarcinoma
• B Squamous cell carcinoma
• C Small cell carcinoma ✓
• D Large cell carcinoma

Explanation

Small cell carcinoma causes the syndrome of inappropriate antidiuretic hormone secretion (SIADH), producing euvolemic hyponatremia with concentrated urine and elevated urine sodium. Squamous cell carcinoma causes hypercalcemia via PTHrP secretion. Adenocarcinoma is associated with hypertrophic pulmonary osteoarthropathy.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.