This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Sex-Hormone, Subject name: Pharmacology
Slidenafil acts by blocking which enzyme? 
Select and explain. 
A. Phosphodiesterase 3 inhibition, B. Phosphodiesterase 5 inhibition, C. 5 alpha Reductase inhibition, D. Stimulate androgen production
B: Phosphodiesterase 5 inhibition
Answer: B: Phosphodiesterase 5 inhibition 
Explanation: Ans. (b) Phosphodiesterase 5 inhibitionRef. KD Tripathi 6th Ed. 1294-295* Sildenafil, Tadalafil and vardenafil are selective PDE-5 inhibitors effective in erectile dysfunction.* 5 a - REDUCTASE INHIBITOR# Finasteride is competitive inhibitor of the enzyme 5a-reductase which converts testosterone into more active dihydrotestosterone responsible for androgen action in many tissues including prostate gland and hair follicles.# Used in treatment of BPH* Flutamide: A nonsteroidal drug having specific antiandrogenic, but no other hormonal activity.# Used in prostate CAAlso KnowTadalafil is the longest acting phospodiesterase inhibitor.