This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Mechanism of action of sildenafil is: March 2012 
Select and explain. 
A. 5 alpha reductase inhibitor, B. Antiandrogen, C. Phosphodiesterase-5 inhibitor, D. Androgenic
C: Phosphodiesterase-5 inhibitor
Answer: C: Phosphodiesterase-5 inhibitor 
Explanation: Ans: C i.e. Phosphodiesterase-5 inhibitor Drugs and mechanism of action Finsateride is a competitive inhibitor of the enzyme 5 alpha reductase (which conves testosterone into more active dihydrotestosterone) Flutamide is a non-steroidal drug having specific anti-androgenic, but no other hormonal activity Sildenafil acts by selectively inhibiting PDE-5 and enhancing nitric oxide action in corpus cavernosum Danazol is an orally active ethisterone derivative having weak androgenic, anabolic and progestational activities.