**Core Concept**
Bisacodyl is a medication used to treat constipation, and its mechanism of action is related to its effect on the gastrointestinal tract. It is a **stimulant laxative**, which works by increasing the amount of fluid in the intestines and stimulating the intestinal muscles to move stool through the digestive system. This action is mediated by the **enteric nervous system**.

**Why the Correct Answer is Right**
Bisacodyl works by directly stimulating the **colonic mucosa**, which leads to an increase in **peristalsis** and water secretion into the colon. This results in softening of the stool and ease of passage. The exact mechanism involves the activation of **guanylate cyclase**, leading to increased levels of **cyclic GMP**, which in turn stimulates the muscularis mucosae to contract.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while bisacodyl does affect bowel movements, its primary action is not as a bulk-forming agent.
**Option B:** Incorrect as bisacodyl does not work by absorbing water to soften the stool.
**Option C:** Incorrect because bisacodyl's action is more direct on the intestinal mucosa rather than solely through the enteric nervous system as a laxative.

**Clinical Pearl / High-Yield Fact**
Bisacodyl is often used to prepare the bowel for colonoscopy or other procedures requiring a clean colon. It's crucial to remember that bisacodyl should not be taken with **antacids** as this can lead to premature dissolution of the enteric coating, reducing its efficacy.

**Correct Answer:** D. Stimulant laxative.