**Core Concept**
Finasteride is a 5-alpha-reductase inhibitor used in the treatment of benign prostatic hyperplasia (BPH) and androgenetic alopecia (male pattern baldness). It works by inhibiting the conversion of testosterone to dihydrotestosterone (DHT), a potent androgen responsible for the growth of prostate tissue and hair loss.

**Why the Correct Answer is Right**
Finasteride specifically inhibits the type 2 isoenzyme of 5-alpha-reductase, which is primarily responsible for converting testosterone to DHT in the prostate and scalp. This leads to a decrease in DHT levels, resulting in a reduction in prostate size and an improvement in symptoms of BPH, as well as an increase in hair growth and a decrease in hair loss. The inhibition of 5-alpha-reductase also has a role in the treatment of hirsutism and acne in women.

**Why Each Wrong Option is Incorrect**
**Option A:** Finasteride is not a selective estrogen receptor modulator (SERM).
**Option B:** Finasteride is not an androgen receptor antagonist.
**Option C:** Finasteride is not an anti-diabetic medication.

**Clinical Pearl / High-Yield Fact**
Finasteride can cause persistent erectile dysfunction and decreased libido in some patients, which can be irreversible in rare cases. Patients should be warned about these potential side effects before starting treatment.

**Correct Answer:** D. 5-alpha-reductase inhibitor.