**Core Concept**
Minoxidil is a vasodilator medication used to treat high blood pressure and alopecia (hair loss). Its therapeutic effects are primarily due to its ability to relax smooth muscle cells in the arterioles, leading to vasodilation and decreased peripheral resistance.

**Why the Correct Answer is Right**
Minoxidil's therapeutic use is as a topical application for hair growth, particularly in the treatment of androgenetic alopecia. This is because minoxidil's vasodilatory effects increase blood flow to the scalp, promoting hair growth. The exact mechanism is not fully understood, but it is thought to involve the stimulation of hair follicle growth and prolongation of the anagen phase.

**Why Each Wrong Option is Incorrect**
**Option A:** Hypotension is a common side effect of minoxidil due to its vasodilatory effects, but it is not its therapeutic use.
**Option B:** Tachycardia may occur as a side effect of minoxidil due to reflex tachycardia secondary to vasodilation, but it is not its therapeutic use.
**Option C:** Hypertrichosis (excessive hair growth) is a side effect of minoxidil, particularly when used orally, but it is not its therapeutic use.
**Option D:** Edema is a side effect of minoxidil, likely due to increased capillary permeability and fluid retention, but it is not its therapeutic use.

**Clinical Pearl / High-Yield Fact**
Minoxidil is unique in that its therapeutic effect (hair growth) is a side effect when used systemically, but is intentionally used as a therapeutic effect when applied topically.

**Correct Answer: C. Hypertrichosis.**