**Core Concept**
K+ channels are a type of ion channel in the cell membrane that allow potassium ions to flow out of the cell. K+ channel openers are a class of drugs that activate these channels, leading to a decrease in intracellular potassium concentration and subsequent relaxation of smooth muscle cells. This mechanism is particularly useful in treating conditions like hypertension and erectile dysfunction.

**Why the Correct Answer is Right**
Minoxidil is a K+ channel opener that acts by binding to and activating ATP-sensitive K+ channels (K_ATP channels) on smooth muscle cells. This activation leads to an increase in potassium efflux, causing hyperpolarization of the cell membrane and relaxation of vascular smooth muscle. As a result, minoxidil dilates blood vessels and decreases blood pressure. Its ability to open K+ channels is the primary mechanism behind its antihypertensive effects.

**Why Each Wrong Option is Incorrect**
**Option A:** Nifedipine is actually a calcium channel blocker, not a K+ channel opener. It inhibits the influx of calcium ions into smooth muscle cells, leading to vasodilation and a decrease in blood pressure.

**Option C:** Enalapril is an angiotensin-converting enzyme (ACE) inhibitor, which blocks the conversion of angiotensin I to angiotensin II. This action reduces vasoconstriction and decreases blood pressure, but it does not involve opening K+ channels.

**Option D:** Atenolol is a beta-blocker that inhibits the effects of epinephrine on beta-adrenergic receptors, leading to decreased heart rate and blood pressure. It does not open K+ channels.

**Clinical Pearl / High-Yield Fact**
Minoxidil is often used to treat resistant hypertension and erectile dysfunction. It is also used topically to treat alopecia areata, a condition characterized by hair loss.

**✓ Correct Answer: B. Minoxidil**