Cholera toxin –
**Core Concept**
Cholera toxin is a potent enterotoxin produced by Vibrio cholerae that causes a severe diarrheal disease, cholera. It acts by stimulating adenylate cyclase activity in intestinal epithelial cells, leading to an increase in cyclic AMP (cAMP) levels.
**Why the Correct Answer is Right**
Cholera toxin binds to GM1 gangliosides on the surface of intestinal epithelial cells and triggers a conformational change in the toxin, which leads to the activation of adenylate cyclase. This results in an increase in cAMP levels, which in turn activates protein kinase A (PKA). PKA phosphorylates and activates the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, leading to an increase in chloride secretion into the intestinal lumen. This results in a massive efflux of water and electrolytes into the lumen, causing severe diarrhea.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cholera toxin does not act by inhibiting the Na+/K+-ATPase pump, which is not directly involved in the pathogenesis of cholera.
**Option B:** This option is incorrect because cholera toxin does not act by stimulating the release of acetylcholine from parasympathetic neurons, which would lead to increased intestinal motility, not diarrhea.
**Option C:** This option is incorrect because cholera toxin does not act by inhibiting the activity of the enzyme, aminopeptidase N, which is not directly involved in the pathogenesis of cholera.
**Clinical Pearl / High-Yield Fact**
Cholera toxin's ability to increase cAMP levels and activate CFTR chloride channels is a classic example of a G-protein coupled receptor-mediated signaling pathway. This mechanism is also involved in other diseases, such as cystic fibrosis.
**Correct Answer:** C.