This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Hematology and CVS, Subject name: Pharmacology
A patient of prinzmetal's angina is staed on isosorbide mononitrate. What is the mechanism of action of nitrates in this condition? 
Select and explain. 
A. Reduced cardiac contractility, B. Increased left ventricular end diastolic volume, C. Decreased diastolic perfusion pressure, D. Endothelium independent coronary vasodilation
D: Endothelium independent coronary vasodilation
Answer: D: Endothelium independent coronary vasodilation 
Explanation: Prinzmetal's angina is due to spasm of main coronary aery. Treatment is by dilatation of coronary aery. Endothelium normally generates NO, but here we are giving nitrates for generation of NO. Mechanism of action of nitrates: Preload reduction >> Afterload reduction Redistribution of coronary flow in the aerial tree action may cause orable redistribution of blood flow to ischaemic areas in angina patients.