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: Pathology
[{'content': 'Which of the following is an impoant mediator of vasodilation? \nSelect and explain. \nA. Nitric oxide, B. Oxygen, C. Leukotriene, D. All of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Vasodilation is induced by the action of several mediators, notably histamine and nitric oxide (NO), on vascular smooth muscle. Ref: Robbins 8th edition, Chapter 2.', 'role': 'assistant'}]