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
[{'content': 'Mechanism of action of sildenafil is \nSelect and explain. \nA. Inhibits PDE 2, B. Inhibits PDE 4, C. Inhibits PDE 5, D. Inhibits PDE 3', 'role': 'user'}, {'content': "Answer: C \nExplanation: Ans. is 'c' i.e., Inhibits PDE 5 * Sildenafil (Viagra) acts to increase cGMP by inhibiting its breakdown by phosphodiesterase isoform 5 (PDE-5).", 'role': 'assistant'}]