This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal System, Subject name: Physiology
Activation of which of the following hormone's receptors produces E.Coli induced diarrhoea? 
Select and explain. 
A. Ghrelin, B. Guanylin, C. VIP, D. Somatostatin
B: Guanylin
Answer: B: Guanylin 
Explanation: Guanylin is a gastrointestinal polypeptide that binds to guanylyl cyclase. It is secreted by cells of the intestinal mucosa. Stimulation of guanylyl cyclase increases the concentration of intracellular cyclic 3',5'-guanosine monophosphate (cGMP), and this in turn causes increased secretion of Cl- into the intestinal lumen. The heat-stable enterotoxin of ceain diarrhea-producing strains of Escherichia coli has a structure very similar to guanylin and activates guanylin receptors in the intestine.