A patient visits his physician for follow-up care related to his long-standing hypertension and to find out why his ankles are swollen. He has been taking an antihypertensive agent for several years, and today his physician tells him that his blood pressure is very high. The physician prescribes hydrochlorothiazide to help remove the fluid from the patient’s legs and to help reduce his blood pressure. Why would the physician prescribe two drugs to treat the patient’s hypertension?