This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
The primary action of Nitric oxide (NO) in the gastrointestinal tract is 
Select and explain. 
A. Vasodilatation, B. Vasoconstriction, C. Gastrointestinal smooth muscle relaxation, D. Gastrointestinal slow smooth muscle contraction
C: Gastrointestinal smooth muscle relaxation
Answer: C: Gastrointestinal smooth muscle relaxation 
Explanation: In the gastrointestinal tract,NO impoant in the relaxation of smooth muscle NO act as inhibitory neurotransmitter in git. It doesnt have membrane bound receptor and diffuses into target cells and iactivates guanyl cyclase- produces cGMP from GTP- mediates smooty muscles relaxation.