**Core Concept**
5α reductase inhibitors are a class of medications that block the enzyme 5α reductase, which is involved in the conversion of testosterone to dihydrotestosterone (DHT). DHT is a potent androgen that plays a key role in the development of benign prostatic hypertrophy (BPH) and male pattern baldness.

**Why the Correct Answer is Right**
Finasteride is a competitive inhibitor of 5α reductase type II, which is the primary enzyme responsible for converting testosterone to DHT in the prostate gland and scalp. By inhibiting this enzyme, finasteride reduces DHT levels, leading to a decrease in prostate size and an improvement in symptoms of BPH, as well as a slowing of hair loss and an increase in hair growth in patients with male pattern baldness.

**Why Each Wrong Option is Incorrect**
**Option A:** Flutamide is a nonsteroidal antiandrogen that blocks the action of androgens, but it does not inhibit 5α reductase and is not used for the treatment of BPH or male pattern baldness.
**Option C:** Prazosin is an alpha-1 adrenergic receptor antagonist that is used to treat hypertension and BPH, but it does not affect 5α reductase activity and is not effective for male pattern baldness.
**Option D:** Minoxidil is a vasodilator that is applied topically to the scalp to stimulate hair growth, but it does not inhibit 5α reductase and is not effective for BPH.

**Clinical Pearl / High-Yield Fact**
Finasteride is a classic example of a medication that has a dual indication for a condition (BPH) and a cosmetic condition (male pattern baldness), highlighting the importance of understanding the underlying pathophysiology of these conditions in order to select the most effective treatment.

**✓ Correct Answer:** B. Finasteride