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
Nitric oxide is produced in 
Select and explain. 
A. Endothelium, B. Plasma, C. Platelets, D. Serum
A: Endothelium
Answer: A: Endothelium 
Explanation: Nitric oxide is formed from Arginine, released from endothelial cells and acts through cGMP to cause vasodilation. 
It is also believed to be involved in gastrointestinal smooth muscle relaxation by acting as a neurotransmitter.