**Core Concept**
Ulcerative colitis (UC) is a chronic inflammatory bowel disease (IBD) characterized by diffuse inflammation and ulceration of the colonic mucosa. The etiology of UC is multifactorial, involving a complex interplay between genetic predisposition, immune system dysregulation, environmental factors, and luminal contents.

**Why the Correct Answer is Right**
The correct answer is D. Rectum. Ulcerative colitis typically begins in the rectum and extends proximally in a continuous manner to involve the entire colon. The rectum is the most common site of disease initiation, and the inflammation often starts in the rectosigmoid region. The rectum's location and the continuous nature of the inflammation are due to the lack of a clear demarcation between the rectum and the rest of the colon, as well as the presence of a high concentration of goblet cells in the rectal mucosa.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because UC does not typically start in the stomach or small intestine, which are more commonly affected by other types of IBD, such as Crohn's disease.
* **Option B:** This is incorrect because UC does not usually start in the duodenum or jejunum, which are parts of the small intestine.
* **Option C:** This is incorrect because UC can involve the entire colon, but it typically starts in the rectum rather than the cecum.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that UC can be classified into three main types: pancolitis (involvement of the entire colon), left-sided colitis (involvement of the rectum and sigmoid colon), and proctitis (involvement of the rectum alone). Understanding the pattern of involvement is crucial for diagnosis and treatment planning.

**Correct Answer: D. Rectum**