This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: CHF, Angina Pectoris and Myocardial Infarction, Subject name: Pharmacology
[{'content': 'You decide not to prescribe sildenafil in a patient because the patient told you that he is taking an antianginal drug. Which of the following can it be? \nSelect and explain. \nA. Calcium channel blockers, B. b adrenergic blockers, C. Organic nitrates, D. Angiotensin conveing enzyme inhibitors', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Nitrates act by increasing cGMP and sildenafil inhibits the breakdown of this compound (by inhibiting phosphodiesterase). Marked increase in cGMP levels may result in the profound hypotension and reflex tachycardia.', 'role': 'assistant'}]