<prompt> Slidenafil acts by blocking which enzyme?Phosphodiesterase 3 inhibitionPhosphodiesterase 5 inhibition5 alpha Reductase inhibitionStimulate androgen production  <response>: 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. 