In a patient with suspected ulcerative colitis (UC) presenting with bloody diarrhea, which part of the bowel is most likely to be affected?

Bowel Involvement in Ulcerative Colitis

In a patient with suspected ulcerative colitis presenting with bloody diarrhea, the rectum is most likely to be affected (option C), as ulcerative colitis characteristically begins in the rectum and extends proximally in a continuous fashion.

Pathophysiology of Ulcerative Colitis

Ulcerative colitis (UC) is a chronic inflammatory bowel disease with a distinct pattern of involvement:

• UC characteristically begins in the rectum and extends proximally in a continuous, confluent, and concentric fashion 1
• The inflammation is limited to the mucosa and occasionally submucosa, unlike Crohn's disease which can be transmural 1
• The demarcation between inflamed and normal areas is usually clear and may occur abruptly 1

Diagnostic Features of Ulcerative Colitis

Endoscopic Findings

• Continuous inflammation starting from the rectum with proximal extension 1
• Loss of normal vascular pattern, granularity, erosions, friability, bleeding, and ulcerations 2
• Distinct demarcation between inflamed and non-inflamed bowel 2

Histological Features

• Diffuse (continuous) crypt architectural irregularity 1
• Diffuse chronic inflammation without skip lesions 1
• Crypt abscesses are common 1
• Mucin depletion is pronounced 1

Distribution Patterns in Ulcerative Colitis

The extent of disease in UC follows a predictable pattern:

1. Proctitis: Limited to the rectum
2. Left-sided colitis: Extends from rectum to splenic flexure (includes sigmoid colon)
3. Extensive colitis/Pancolitis: Extends beyond the splenic flexure, potentially involving the entire colon

According to the Montreal classification for UC 1:

• E1: Ulcerative proctitis (rectum only)
• E2: Left-sided UC (distal to splenic flexure)
• E3: Extensive UC (proximal to splenic flexure)

Atypical Patterns

While the rectum is almost always involved, some exceptions exist:

• Rectal sparing has been described in up to >3% of patients, more frequently in those who have received empirical topical therapy 1
• "Caecal patch" (isolated periappendiceal inflammation) can occur in patients with left-sided colitis 1
• "Backwash ileitis" (inflammation of terminal ileum) can occur in approximately 20% of patients with extensive colitis 1

Clinical Implications

The extent of inflammation in UC has prognostic implications:

• The 10-year colectomy rate is 19% for extensive colitis, 8% for left-sided colitis, and 5% for proctitis 1
• Treatment approaches differ based on disease extent, with topical therapies often sufficient for proctitis but more extensive disease requiring systemic treatments 2

Conclusion

In a patient with suspected ulcerative colitis presenting with bloody diarrhea, the rectum (option C) is the most likely site to be affected, as UC characteristically begins in the rectum and extends proximally in a continuous fashion. This is in contrast to Crohn's disease, which can have a patchy distribution and may spare the rectum 3.