Hyponatraemia & Hypernatraemia — Practice

A 62-year-old man with a 40-pack-year smoking history presents with hyponatraemia (serum sodium 118 mEq/L), serum osmolality 258 mOsmol/kg, urine osmolality 520 mOsmol/kg, and urine sodium 65 mEq/L. He is clinically euvolaemic. Chest X-ray shows a hilar mass. What is the most likely diagnosis?