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 a potent vasodilator. Where is it produced from? 
Select and explain. 
A. Endothelium, B. RBC, C. Platelets, D. Lymphocytes
A: Endothelium
Answer: A: Endothelium 
Explanation: Nitric oxide, known as the 'endothelium-derived relaxing factor', or 'EDRF'. Endothelial cells contain the constitutive form of nitric oxide synthase, which produces moderate amounts of nitric oxide by metabolizing L-arginine. The endothelium of blood vessels uses nitric oxide to signal the surrounding smooth muscle to relax, thus resulting in vasodilation and increasing blood flow. Functions: Endothelium-derived nitric oxide inhibits contraction and proliferation of the underlying vascular smooth muscle, adhesion of blood cells and platelets, and platelet aggregation. Ref: Hypeension Primer: The Essentials of High Blood Pressure By Joseph L Izzo, Jr, Henry R Black, M.D., Theodore L. Goodfriend, 2003, Page 44; Guyton's physiology, 22nd edition, Page 199