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
[{'content': 'Nitric oxide is synthesised from: \nSelect and explain. \nA. Cysteine, B. Cystine, C. Methionine, D. Arginine', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 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.', 'role': 'assistant'}]