## Core Concept
The question requires the identification of Drug A based on its mechanism of action. The mechanism involves the inhibition of angiotensin-converting enzyme (ACE), which is a key component of the renin-angiotensin-aldosterone system (RAAS) that regulates blood pressure.

## Why the Correct Answer is Right
The correct answer, **Captopril**, is right because it is an ACE inhibitor. ACE inhibitors work by blocking the conversion of angiotensin I to angiotensin II, a potent vasoconstrictor. This blockade results in decreased levels of angiotensin II, leading to vasodilation and a decrease in blood pressure. Captopril is a well-known ACE inhibitor used primarily in the treatment of hypertension and heart failure.

## Why Each Wrong Option is Incorrect
* **Option A:** **Nifedipine** is a calcium channel blocker, not an ACE inhibitor. It works by inhibiting the influx of calcium ions into vascular smooth muscle cells and cardiac muscles, causing vasodilation and a decrease in blood pressure.
* **Option B:** **Propranolol** is a beta-blocker. It decreases blood pressure by blocking beta-1 and beta-2 adrenergic receptors, reducing heart rate, and decreasing the force of contraction of the heart.
* **Option D:** **Furosemide** is a loop diuretic. It acts by inhibiting the sodium-potassium-chloride cotransporter in the thick ascending limb of the loop of Henle, leading to increased sodium and water excretion and a decrease in blood pressure.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that ACE inhibitors like **Captopril** are particularly beneficial in patients with heart failure, diabetes, or chronic kidney disease, as they provide protective effects on the kidneys and heart beyond just lowering blood pressure.

## Correct Answer: C. Captopril