A 61-year-old man presented to the hospital with shortness of breath, decreased exercise capacity, and distended neck veins. The man had a history of severe hypertension currently treated with hydrochlorothiazide and captopril. Despite the treatment, his blood pressure was still 170/110. After physical examination and laboratory tests, a diagnosis of diastolic cardiac failure was made. Which of the following would be a useful drug to add to the patient’s therapeutic regimen?