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': 'Mechanism of action of cholera toxin: September 2007, 2009 \nSelect and explain. \nA. Increase in c-AMP, B. Decrease in c-AMP, C. Activates Na+ K+ ATPase, D. Inhibit Na+ K+ ATPase', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. A: Increase in c-AMP The debilitating loss of intestinal fluid is caused primarily by the release of cholera toxin, the main virulence factor of the pathogen Vibrio cholerae. Cholera toxin consists of an A subunit coupled to a B subunit: the A subunit consists of an Al domain containing the enzymatic active site, and an A2 domain while the B subunit contains five identical peptides that assemble into a pentameric ring surrounding a central pore. This AB; structure is closely related to the heat-labile enterotoxin (LT) from enterotoxigenic Escherichia coli, which causes diarrhoea, often in infants. Symptoms begin when the massive secretion of water and sodium in the small intestine, a consequence of the activation of cyclic AMP, exceeds the resorptive capacity of the colon.', 'role': 'assistant'}]