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': 'Nitric oxide is produced in \nSelect and explain. \nA. Endothelium, B. Plasma, C. Platelets, D. Serum', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Nitric oxide is formed from Arginine, released from endothelial cells and acts through cGMP to cause vasodilation. \nIt is also believed to be involved in gastrointestinal smooth muscle relaxation by acting as a neurotransmitter.', 'role': 'assistant'}]