**Core Concept**
Ulcerative colitis is a type of inflammatory bowel disease (IBD) characterized by chronic inflammation and ulceration of the colon's mucosal lining. It is a condition where the immune system mistakenly attacks the healthy cells in the colon, leading to symptoms such as diarrhea, abdominal pain, and rectal bleeding.

**Why the Correct Answer is Right**
The correct answer is **A. Rectum**. Ulcerative colitis typically starts in the rectum and extends proximally in a continuous manner, often affecting the entire colon. This is because the rectum is the most distal portion of the colon, and the inflammation tends to spread upwards. The pathophysiology involves an abnormal immune response to luminal antigens, leading to the release of pro-inflammatory cytokines and the activation of various immune cells, including T cells and macrophages.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because while the sigmoid colon can be involved in ulcerative colitis, it is not the typical starting point. The sigmoid colon is a common site for inflammation, but it is usually part of the more extensive colonic involvement.
**Option C:** This option is incorrect because the cecum is often spared in ulcerative colitis, especially in the early stages. The cecum is a common site for Crohn's disease, a different type of IBD.
**Option D:** This option is incorrect because the small intestine is not typically involved in ulcerative colitis. Ulcerative colitis is a colonic disease that rarely extends beyond the colon.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of ulcerative colitis is the presence of **continuous mucosal inflammation**, which starts in the rectum and extends proximally. This can be distinguished from Crohn's disease, which can affect any part of the gastrointestinal tract and often presents with **skip lesions**.

**Correct Answer: A. Rectum**