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: Medicine
Agent of first choice in an acute attack of Prinzmetal's angina is: 
Select and explain. 
A. Diltiazem, B. Nitrates, C. Propranolol, D. Verapamil
B: Nitrates
Answer: B: Nitrates 
Explanation: Answer is B (Nitrates) "Sublingual nitroglycerine is the drug of choice" -- CMDT Nitrates decrease aeriolar and venous tone, reduce preload and afterload, and reduce the oxygen demand of the hea. Nitrates may also improve myocardial blood flow by dilating collateral channels in the presence of increased vasomotor tone, or coronary stenosis.