**Core Concept**
ACE inhibitors are a class of medications used to treat hypertension and heart failure by blocking the conversion of angiotensin I to angiotensin II, a potent vasoconstrictor. Some ACE inhibitors are prodrugs, requiring metabolic activation to produce their active form.

**Why the Correct Answer is Right**
In contrast to other ACE inhibitors, **Captopril** is not a prodrug. It is administered in its active form and does not require metabolic conversion to exert its effects. Prodrugs, on the other hand, require enzymatic conversion to their active form before exerting their pharmacological effects. This is in contrast to **Enalapril**, which is a prodrug that is metabolized to its active form, **Enalaprilat**.

**Why Each Wrong Option is Incorrect**
**Option A:** Enalapril is a prodrug that requires metabolic conversion to its active form, Enalaprilat.
**Option B:** Lisinopril is also a prodrug that is metabolized to its active form before exerting its effects.
**Option D:** Quinapril is another prodrug that requires metabolic activation to produce its active form.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that not all ACE inhibitors are prodrugs, and Captopril is the only one that is administered in its active form.

**Correct Answer: C. Captopril**