This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Pharmacology
Mechanism of action of finasteride is - 
Select and explain. 
A. Androgen receptor antagonist, B. 5 - a reductase inhibitor, C. 17 - a hydroxylase inhibitor, D. Aromatase inhibitor
B: 5 - a reductase inhibitor
Answer: B: 5 - a reductase inhibitor 
Explanation: Ans. is 'b' i.e., 5-a reductase inhibitor Finasteride* Competitive inhibitor of enzyme 5 a reductase, which converts testosterone to more active DHT (Di hydro testosterone) which is responsible for majority androgenic actions, including the actions on prostate and hair follicles.* It is relatively selective for 5 a reductase type 2 isoenzyme which predominates in male urogenital tract, and is used in BPH, i.e., Benign prostatic hyperplasia to shrink the prostatic size and increase peak urinary flow rate.* It has also been found effective in male pattern baldness.Adverse effects* Decreased libido, impotence, decreased volume of ejaculate.* Gynecomastia, skin rashes, lip swelling may occur.