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
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. β adrenergic blockers, C. Organic nitrates, D. Angiotensin converting enzyme inhibitors
C: Organic nitrates
Answer: C: Organic nitrates 
Explanation: None