Finasteride acts by blocking
**Core Concept**
Finasteride is a 5-alpha-reductase inhibitor, a class of drugs 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 that plays a key role in the development of these conditions.
**Why the Correct Answer is Right**
Finasteride specifically targets the type II isoform of 5-alpha-reductase, an enzyme responsible for converting testosterone to DHT in the prostate gland and scalp. By blocking this enzyme, finasteride reduces the levels of DHT, leading to a decrease in prostate size and an improvement in hair growth. This is particularly beneficial for patients with BPH, as it helps to alleviate symptoms such as urinary frequency and nocturia.
**Why Each Wrong Option is Incorrect**
**Option A:** Androgen receptors - Finasteride does not act by blocking androgen receptors, but rather by inhibiting the production of DHT, which then has reduced affinity for androgen receptors.
**Option B:** 17-alpha-hydroxylase - This enzyme is involved in the synthesis of cortisol and other glucocorticoids, and is not the target of finasteride.
**Option C:** Aromatase - This enzyme is responsible for converting androgens to estrogens, and is not relevant to the mechanism of action of finasteride.
**Clinical Pearl / High-Yield Fact**
Finasteride is a prodrug that requires metabolism to its active form, finasteride-5-alpha-reductase complex, before it can exert its effects. This is an important consideration when interpreting laboratory results, as the levels of finasteride in the blood may not accurately reflect its therapeutic efficacy.
**Correct Answer: C. 5-alpha-reductase**