How does Crohn's disease (Crohn's disease) differ from ulcerative colitis (ulcerative colitis) in terms of rectal involvement?

Differences Between Crohn's Disease and Ulcerative Colitis: Rectal Involvement

In contrast to ulcerative colitis, Crohn's disease may spare the rectum (option b is correct). While ulcerative colitis almost always involves the rectum in untreated patients, Crohn's disease characteristically shows a patchy, discontinuous distribution that can skip the rectum entirely 1.

Anatomical Distribution Patterns

• Ulcerative colitis (UC) typically begins in the rectum and extends proximally in a continuous fashion with gradually decreasing severity of inflammation 1
• Crohn's disease (CD) shows a patchy, discontinuous distribution with skip lesions throughout the gastrointestinal tract 1
• Rectal involvement is almost always present in untreated UC (>97% of cases), though therapy can sometimes alter this pattern 2, 1
• Rectal sparing is a key distinguishing feature of Crohn's disease, occurring commonly, whereas it is rare in UC (described in only up to 3% of UC patients) 2, 3

Depth of Inflammation and Tissue Involvement

• UC inflammation is limited to the mucosa and occasionally submucosa, while CD inflammation is transmural (affecting all layers of the intestinal wall) 1
• In UC, fibrosis is commonly restricted to mucosa or submucosa, whereas in CD fibrosis can extend through all layers 1
• Perianal fistulas and ulcers are rare in UC but common in CD 3

Microscopic Features and Diagnostic Criteria

• Granulomas (non-cryptolytic) are absent in UC but present in CD, serving as a key distinguishing feature 1, 3
• Crypt abscesses are more common in UC (41%) than in CD (19%) 1
• The inflammatory infiltrate in UC is diffuse without variations in intensity, while in CD it varies in intensity within and between biopsies 1

Addressing Other Options in the Question

• Cancer risk: Both UC and CD carry an increased risk of colorectal cancer, particularly with longstanding disease 3
• Sclerosing cholangitis: Primary sclerosing cholangitis is more commonly associated with UC than with CD 2
• Toxic megacolon: This severe complication can occur in both diseases but is more commonly associated with UC rather than CD 4

Clinical Implications of Rectal Sparing

• The presence of rectal sparing strongly suggests Crohn's disease rather than UC in a patient with inflammatory bowel disease 2, 3
• This distinction is clinically important as treatment approaches and prognosis differ between the two conditions 3
• In cases where rectal sparing is observed in a patient diagnosed with UC, it's important to consider whether this is due to topical therapy effects or if the diagnosis should be reconsidered 2, 5

Diagnostic Challenges

• In 5-15% of IBD patients, endoscopic and histological assessments cannot distinguish between Crohn's colitis and UC, leading to a classification of IBD-unclassified (IBD-U) 2, 1
• Full ileocolonoscopy is recommended to delineate disease extent, severity of inflammation, and to distinguish between UC and CD 2
• Absolute rectal sparing (complete absence of histologic features of chronicity or activity in the rectum) is extremely rare in UC even after long-term use of anti-inflammatory medications 5

The presence of rectal sparing is therefore a key distinguishing feature that points toward Crohn's disease rather than ulcerative colitis, making option (b) the correct answer.