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: Unknown
All of the following are obligate coronary vasodilators except: 
Select and explain. 
A. Nitroglycerine, B. Nitric oxide, C. Hypoxia and Hypercapnia, D. Acetylcholine
D: Acetylcholine
Answer: D: Acetylcholine 
Explanation: All are known to cause vasodilatation.
Only acetylcholine can also be a vasoconstrictor if the endothelium is dysfunctional or absent and nitric oxide can not be produced. In that case it is not an obligate vasodilator.