A 49-year-old man with a history of hypertension controlled with diet and exercise now has a blood pressure of 160/90 mm Hg. His physician begins treatment with a single agent. However, the patient now complains of tiredness and cannot jog more than 4 miles. In the past, he was able to run a half marathon (13.1 miles). Which one of the following drugs is he most likely to be taking for hypertension?