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: Pathology
Nitric oxide is synthesised from: 
Select and explain. 
A. Cysteine, B. Cystine, C. Methionine, D. Arginine
D: Arginine
Answer: D: Arginine 
Explanation: NO is synthesized from l-arginine by the enzyme nitric oxide synthase (NOS). There are three different types of NOS: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS). eNOS and nNOS are constitutively expressed at low levels and can be activated rapidly by an increase in cytoplasmic Ca2+. iNOS, in contrast, is induced when macrophages and other cells are activated by cytokines (e.g., TNF, IFN-g) or microbial products. Ref: Robbins 8th edition, Chapter 2.