This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous (Pharmacology), Subject name: Pharmacology
Mechanism of action of sildenafil is 
Select and explain. 
A. Inhibits PDE 2, B. Inhibits PDE 4, C. Inhibits PDE 5, D. Inhibits PDE 3
C: Inhibits PDE 5
Answer: C: Inhibits PDE 5 
Explanation: Ans. is 'c' i.e., Inhibits PDE 5 * Sildenafil (Viagra) acts to increase cGMP by inhibiting its breakdown by phosphodiesterase isoform 5 (PDE-5).