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
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? 
Select and explain. 
A. Calcium channel blockers, B. b adrenergic blockers, C. Organic nitrates, D. Angiotensin conveing enzyme inhibitors
C: Organic nitrates
Answer: C: Organic nitrates 
Explanation: 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.