**Core Concept**
Ulcerative colitis (UC) is a type of inflammatory bowel disease (IBD) characterized by chronic inflammation and ulcers in the colon. The disease typically starts in the rectum and extends proximally in a continuous manner.

**Why the Correct Answer is Right**
Ulcerative colitis almost always involves the rectum due to the unique anatomy of the rectum. The rectum is the terminal portion of the large intestine, and its position and blood supply make it more susceptible to inflammation. The rectum's proximity to the anus also facilitates the spread of inflammation to the entire colon. This is in contrast to other forms of IBD, such as Crohn's disease, which can affect any part of the gastrointestinal tract.

**Why Each Wrong Option is Incorrect**
**Option A:** Ulcerative colitis can sometimes involve the terminal ileum, but this is not the typical pattern. The terminal ileum is more commonly affected in Crohn's disease.
**Option B:** Ulcerative colitis does not typically involve the small intestine. While some cases may extend into the ascending colon, the disease generally spares the small intestine.
**Option C:** Ulcerative colitis can involve the entire colon, but this is not the most accurate description. The disease typically starts in the rectum and extends proximally in a continuous manner.

**Clinical Pearl / High-Yield Fact**
The extent of colonic involvement in ulcerative colitis can be remembered using the term "continuous" or "backwash" ileitis, which refers to the spread of inflammation from the rectum to the terminal ileum.

**Correct Answer:** C.