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: Microbiology
[{'content': 'What is the mechanism of action of cholera toxin in the small intestine? \nSelect and explain. \nA. ADP-ribosylation of the G regulatory protein, B. Activation of guanylate cyclase, C. Inhibition of adenyl cyclase, D. Inhibition of protein synthesis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Cholera toxin is an example of A-B model of toxins. B subunit of the toxin binds to the enterocyte receptors, active toxin subunit A activates adenylate cyclase of the intestinal epithelial cells by ADP ribosylation of the Gs subunit of the G protein of intestinal cells. This prevents the G protein from being turned off, and result in excess production of cAMP. cAMP in turn act as a second messenger to activate protein kinase A, which intern activates the specific cell proteins that control secretion of electrolytes. Ref: Textbook of Pathology By V. Krishna, Page 178; Medical Microbiology By Fritz H. Kayse, Page 298; Microbiology and immunology By Monica Gandhi, Page 45', 'role': 'assistant'}]