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
[{'content': 'All of the following are obligate coronary vasodilators except: \nSelect and explain. \nA. Nitroglycerine, B. Nitric oxide, C. Hypoxia and Hypercapnia, D. Acetylcholine', 'role': 'user'}, {'content': 'Answer: D \nExplanation: All are known to cause vasodilatation.\nOnly 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.', 'role': 'assistant'}]