**Core Concept**
Cholera toxin is a potent enterotoxin produced by Vibrio cholerae, the causative agent of cholera. It acts by activating the adenylate cyclase enzyme in intestinal epithelial cells, leading to a massive increase in intracellular cyclic AMP (cAMP) levels. This results in the secretion of large amounts of water and electrolytes into the intestinal lumen, causing the characteristic profuse diarrhea of cholera.

**Why the Correct Answer is Right**
Cholera toxin is composed of two subunits: A and B. The B subunit binds to GM1 gangliosides on the surface of intestinal epithelial cells, while the A subunit is responsible for the enzymatic activity that activates adenylate cyclase. This results in an increase in cAMP levels, which activates protein kinase A (PKA) and leads to the phosphorylation of a protein called the cystic fibrosis transmembrane conductance regulator (CFTR). Phosphorylated CFTR increases chloride ion secretion into the intestinal lumen, drawing water into the lumen and resulting in 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. While cholera toxin does increase intracellular cAMP levels, it does so by activating adenylate cyclase, not by inhibiting phosphodiesterase.

**Option B:** This option is incorrect because it implies that cholera toxin acts by increasing intracellular calcium levels, which is not the case. Cholera toxin actually increases intracellular cAMP levels, not calcium.

**Option C:** This option is incorrect because it implies that cholera toxin acts by inhibiting sodium absorption in the intestine. While cholera toxin does lead to the secretion of sodium and chloride ions into the intestinal lumen, this is a result of the increased chloride ion secretion caused by the activation of CFTR, not a direct inhibition of sodium absorption.

**Clinical Pearl / High-Yield Fact**
The GM1 ganglioside receptor on the surface of intestinal epithelial cells is the target of the B subunit of cholera toxin. This receptor is also the target of other toxins, such as the Shiga toxin produced by Shigella dysenteriae.

**Correct Answer: C. Cholera toxin increases intracellular cAMP levels by activating adenylate cyclase, leading to the secretion of water and electrolytes into the intestinal lumen.**