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': 'The primary action of the neurotransmitter Nitric oxide (NO) in the gastrointestinal tract is: \nSelect and explain. \nA. Vasodilatation, B. Vasoconstriction, C. Gastrointestinal smooth muscle relaxation, D. Gastrointestinal slow smooth muscle contraction', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Nitric oxide (NO) is an inhibitory neurotransmitter of enteric motor neurons that mediates Gastrointestinal smooth muscle relaxation. Unlike most neurotransmitters and hormones, NO does not act a membrane bound receptor. Instead NO readily diffuses into adjacent target cells to directly activate guanylcyclase and mediate smooth muscle relaxation. Ref: Review of Medical Physiology by William ganong, 22nd edn/page 598-9', 'role': 'assistant'}]