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: Physiology
[{'content': 'Which of the following is an endothelial smooth muscle relaxing factor? \nSelect and explain. \nA. Nitric oxide, B. Angiotensin, C. Dopamine, D. Vasopressin', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Nitric oxide acts in paracrine manner on target cells through induction of cGMP which in turn initiates a series of intravascular events leading to relaxation of vascular smooth muscle.', 'role': 'assistant'}]