**Core Concept**
Tenapanor is a small molecule inhibitor that targets the sodium-hydrogen exchanger 3 (NHE3) in the small intestine. NHE3 plays a crucial role in the absorption of sodium ions and the secretion of hydrogen ions, which is essential for maintaining intestinal pH homeostasis.

**Why the Correct Answer is Right**
Tenapanor works by inhibiting NHE3, which leads to a decrease in sodium absorption and an increase in hydrogen secretion. This results in a decrease in intestinal pH, which is beneficial for patients with irritable bowel syndrome with constipation (IBS-C). By inhibiting NHE3, tenapanor also decreases the absorption of sodium and water, leading to a decrease in stool output and an increase in stool frequency.

**Why Each Wrong Option is Incorrect**
* **Option A:** Tenapanor is not a prosthetic agent, which is used to treat conditions like gastroparesis. Its mechanism of action is distinct from prosthetic agents.
* **Option B:** Tenapanor does not work by inhibiting the motilin receptor, which is a different target for gastrointestinal motility disorders.
* **Option C:** Tenapanor is not a bile acid binder, which is used to treat conditions like primary biliary cholangitis. Its mechanism of action is distinct from bile acid binders.

**Clinical Pearl / High-Yield Fact**
Tenapanor is the first-in-class NHE3 inhibitor approved for the treatment of IBS-C. Its unique mechanism of action provides a new therapeutic option for patients with this condition.

**Correct Answer: C. Tenapanor is a small molecule inhibitor of the sodium-hydrogen exchanger 3 (NHE3) in the small intestine.**