In the small intestine, cholera toxin acts by-
## Core Concept
The cholera toxin is a key virulence factor produced by Vibrio cholerae, the bacterium responsible for cholera. It acts on the small intestine, causing a profound effect on intestinal secretion. The toxin's mechanism involves altering the normal signaling pathways within intestinal epithelial cells.
## Why the Correct Answer is Right
The cholera toxin works by specifically targeting the intestinal epithelial cells. It enters the cell through endocytosis and then releases its A subunit, which activates **Gs alpha subunit** of the G protein complex. This activation leads to an increase in intracellular cyclic AMP (cAMP) levels. Elevated cAMP levels cause the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel to open, leading to an increased secretion of chloride ions into the intestinal lumen. Water follows the chloride ions due to osmosis, resulting in the characteristic watery diarrhea of cholera.
## Why Each Wrong Option is Incorrect
- **Option A:** Incorrect because cholera toxin does not primarily act by inhibiting the absorption of sodium and water; rather, it increases the secretion of chloride and water.
- **Option B:** Incorrect as cholera toxin does not directly stimulate the release of prostaglandins to cause its effect, although prostaglandins can also increase cAMP levels and contribute to secretion.
- **Option C:** This seems to be the correct mechanism but let's evaluate all options.
- **Option D:** Incorrect because the toxin does not act by blocking the effect of a specific neurotransmitter that would otherwise decrease secretion.
## Clinical Pearl / High-Yield Fact
A key point to remember is that the cholera toxin's effect is not due to invasion of the intestinal mucosa or production of an inflammatory response but through the action on the adenylate cyclase pathway, leading to increased cAMP and thus secretion. This results in a profuse, watery diarrhea often described as "rice-water stool."
## Correct Answer: C.