**Core Concept**
Cholera toxin is a potent enterotoxin secreted by Vibrio cholerae, a Gram-negative bacterium that causes cholera, a diarrheal disease. The toxin acts by activating adenylate cyclase in intestinal epithelial cells, leading to a massive increase in intracellular cyclic AMP (cAMP) levels.

**Why the Correct Answer is Right**
Cholera toxin works by binding to GM1 gangliosides on the surface of intestinal epithelial cells. This binding activates the toxin's A subunit, which then catalyzes the transfer of ADP-ribose from NAD+ to the alpha subunit of the Gs protein, a G-protein coupled receptor. This modification activates the Gs protein, which in turn stimulates adenylate cyclase to produce cAMP. The increased cAMP levels activate protein kinase A (PKA), leading to the secretion of chloride and water into the intestinal lumen, resulting in profuse diarrhea.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the mechanism of action of cholera toxin.
**Option B:** This option is incorrect because Vibrio cholerae is a Gram-negative bacterium, not a Gram-positive bacterium.
**Option C:** This option is incorrect because it does not accurately describe the pathophysiology of cholera.

**Clinical Pearl / High-Yield Fact**
Cholera toxin is a classic example of a toxin that exploits the host's cellular signaling pathways to cause disease. Understanding the molecular mechanism of cholera toxin is crucial for the development of effective treatments and vaccines against cholera.

**Correct Answer:** D. Vibrio cholerae.