**Question:** The 5a reductase inhibitor that has been found to be effective both in benign prostatic hyperophy and male pattern baldness is:

A. Finasteride
B. Dutasteride
C. Selenium
D. Minoxidil

**Correct Answer:** A. Finasteride

**Core Concept:**
5a reductase inhibitors are a class of medications that work by inhibiting the enzyme 5a reductase, which is responsible for converting testosterone to dihydrotestosterone (DHT) in the body. DHT is a potent androgen that plays a significant role in prostate enlargement and hair follicle atrophy in male pattern baldness.

**Why the Correct Answer is Right:**
Finasteride (correct answer) is a selective inhibitor of type 2 5a reductase, which is predominantly found in the prostate gland. By inhibiting this specific type of 5a reductase, Finasteride reduces the production of DHT, leading to a decrease in prostate size and improvement in symptoms associated with benign prostatic hyperplasia (BPH). In terms of male pattern baldness, Finasteride also reduces DHT levels, thereby slowing down hair follicle atrophy and promoting hair regrowth.

**Why Each Wrong Option is Incorrect:**

A. Dutasteride (wrong answer) is a broader inhibitor of both types 1 and 2 of 5a reductase. Although it has some efficacy in treating BPH, it is not as selective as Finasteride, making Finasteride a more effective choice for treating BPH specifically.

B. Selenium (wrong answer) is an essential trace element involved in various antioxidant functions and immune system regulation. There is no direct evidence indicating selenium's efficacy in treating BPH or male pattern baldness.

C. Minoxidil (wrong answer) is a potassium channel opener used in the treatment of hypertension and hair loss. While it can promote hair regrowth in a few individuals with male pattern baldness, it is not a selective 5a reductase inhibitor and is not effective in treating BPH.

**Clinical Pearl:** Finasteride is a selective and effective medication for treating benign prostatic hyperplasia and male pattern baldness due to its selectivity for type 2 5alpha reductase inhibition, which leads to reduced DHT levels and improved symptoms in both conditions.