Cholera toxin –
**Core Concept**
Cholera toxin is a potent enterotoxin produced by Vibrio cholerae, the bacterium responsible for cholera. It causes a severe diarrheal disease by stimulating the secretion of chloride ions into the intestinal lumen, leading to a massive loss of fluids and electrolytes.
**Why the Correct Answer is Right**
Cholera toxin works by activating the adenylate cyclase enzyme in the intestinal epithelial cells. This activation leads to an increase in intracellular cyclic AMP (cAMP) levels. Elevated cAMP levels stimulate the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels, resulting in the secretion of chloride ions into the intestinal lumen. This chloride secretion is followed by the movement of water into the lumen through osmosis, leading to profuse diarrhea.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cholera toxin does not primarily affect the sodium ion channels in the intestinal epithelial cells. While sodium ion channels do play a role in fluid secretion, the primary mechanism of cholera toxin involves the activation of adenylate cyclase and the subsequent increase in cAMP levels.
**Option B:** This option is incorrect because cholera toxin does not directly stimulate the release of histamine from mast cells. The pathophysiology of cholera involves the activation of adenylate cyclase and the increase in cAMP levels, not the release of histamine.
**Option C:** This option is incorrect because cholera toxin does not primarily affect the smooth muscle contraction in the intestinal tract. While smooth muscle contraction can contribute to the development of diarrhea, the primary mechanism of cholera toxin involves the secretion of chloride ions into the intestinal lumen.
**Option D:** This option is incorrect because cholera toxin does not primarily affect the gut motility. While changes in gut motility can contribute to the development of diarrhea, the primary mechanism of cholera toxin involves the secretion of chloride ions into the intestinal lumen.
**Clinical Pearl / High-Yield Fact**
The oral rehydration solution (ORS) used to treat cholera works by replacing the lost fluids and electrolytes. The ORS contains a mixture of sodium chloride, glucose, and potassium ions, which helps to restore the balance of fluids and electrolytes in the body. The use of ORS has been instrumental in reducing the mortality rate from cholera.
**Correct Answer:** D.