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': 'Mechanism of action of Nitric oxide is through \nSelect and explain. \nA. cGMP, B. cAMP, C. Ca++, D. Tyrosine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Nitric oxide is formed from Arginine, released from endothelial cells and acts through cGMP to cause vasodilation.', 'role': 'assistant'}]