**Core Concept**
Angiotensin-converting enzyme I (ACE I) is a crucial enzyme in the renin-angiotensin-aldosterone system (RAAS), responsible for converting angiotensin I to angiotensin II, a potent vasoconstrictor. Inhibiting ACE I leads to decreased angiotensin II levels, causing vasodilation and a subsequent decrease in blood pressure.

**Why the Correct Answer is Right**
Enalapril is a prodrug that is converted to its active form, enalaprilat, which is a potent inhibitor of ACE I. By inhibiting ACE I, enalaprilat prevents the conversion of angiotensin I to angiotensin II, leading to vasodilation and a decrease in blood pressure. This mechanism is the primary mechanism of action of ACE inhibitors, which are commonly used to treat hypertension and heart failure.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydralazine is a direct vasodilator that acts on the smooth muscle of blood vessels, causing relaxation and decreased blood pressure. It does not inhibit ACE I.

**Option C:** Spironolactone is a potassium-sparing diuretic that acts on the aldosterone receptor, preventing the reabsorption of sodium and water in the collecting duct. It does not inhibit ACE I.

**Option D:** Metoprolol is a beta-blocker that acts on the beta-adrenergic receptors, reducing heart rate and contractility. It does not inhibit ACE I.

**Clinical Pearl / High-Yield Fact**
ACE inhibitors are a cornerstone in the treatment of hypertension and heart failure, and are often used in combination with other medications to achieve optimal blood pressure control.

**Correct Answer:**
✓ Correct Answer: B. enalapril