Which of the following is an ACE inhibitor?

Captopril is indeed classified as an ACE inhibitor. ACE inhibitors are a group of medications that inhibit the angiotensin-converting enzyme, which plays a crucial role in the renin-angiotensin-aldosterone system (RAAS). This action helps to relax blood vessels, lower blood pressure, and reduce the strain on the heart, making ACE inhibitors like captopril effective in treating conditions such as hypertension and heart failure.

In contrast, the other options pertain to different classes of medications. Losartan is an angiotensin II receptor blocker (ARB) that works by preventing angiotensin II from exerting its effects on blood vessels. Hydrochlorothiazide is a thiazide diuretic, primarily used to eliminate excess fluid and lower blood pressure by reducing blood volume. Amlodipine is a calcium channel blocker that helps to relax the muscles of the heart and blood vessels, thereby lowering blood pressure. Although all these medications are used in the management of hypertension, they do not share the same mechanism of action as ACE inhibitors.