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
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: C i.e. Gastrointestinal smooth muscle relaxation - Nitric oxide (NO) is synthesized from arginine (02, NADPH) by NO synthase (requiring FMN, FAD, herne & BH4) in nerves and vascular endothelial cells (hence called EDRF). It relaxes smooth muscles by increasing cyclic GMPQ. - Nitric oxide (NO) and atrial natriuretic factor (ANF) are hormones that act through c-GMPQ. - NO (along with VIP and PACAP) are inhibitory neurotransmitter in enteric (gastrointestinal) neurons that primarily mediate gastrointestinal smooth muscle relaxation c-GMP pathwayQ.