CNS Pharmacology USMLE Step 1 Practice Question

A 59-year-old man with a known history of small cell lung carcinoma, currently undergoing chemotherapy, presents to the emergency department with a 3-day history of progressive confusion, headache, and two witnessed generalized tonic-clonic seizures. His wife reports he has had no vomiting or excessive fluid intake. Vital signs show blood pressure 128/76 mmHg, heart rate 88/min, respiratory rate 16/min, and temperature 37.1°C. Physical examination reveals no peripheral edema, normal skin turgor, and moist mucous membranes consistent with euvolemia. Laboratory testing shows serum sodium 116 mEq/L, serum osmolality 242 mOsm/kg, urine osmolality 520 mOsm/kg, and urine sodium 45 mEq/L. After a trial of strict fluid restriction fails to correct the hyponatremia, he is started on a medication that antagonizes vasopressin V2 receptors in the collecting duct. Which of the following drugs was most likely prescribed?