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
Which of the following is an impoant mediator of vasodilation? 
Select and explain. 
A. Nitric oxide, B. Oxygen, C. Leukotriene, D. All of the above
A: Nitric oxide
Answer: A: Nitric oxide 
Explanation: 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.