Most Likely Diagnosis: IBD Associated with Seronegative Spondyloarthritis (Answer B)
In a 35-year-old patient with established ankylosing spondylitis presenting with mucoid bloody stools, abdominal pain, elevated inflammatory markers, and terminal ileal ulcers on colonoscopy, the most likely diagnosis is IBD associated with seronegative spondyloarthritis, though Crohn's disease remains a strong differential that cannot be excluded until biopsy results are available.
Clinical Reasoning
Why IBD-Associated SpA is Most Likely
The patient's pre-existing ankylosing spondylitis fundamentally changes the diagnostic landscape. This is not a de novo presentation of isolated gastrointestinal disease—it's a patient with established seronegative spondyloarthritis now developing intestinal manifestations.
Between 5-10% of AS patients develop overt IBD (either Crohn's disease or ulcerative colitis), while up to 50% have subclinical gut inflammation 1. The 2024 Italian consensus guidelines specifically recognize "IBD-associated SpA" as a distinct clinical entity where IBD and SpA coexist in the same patient, and importantly note that the occurrence of IBD and SpA may not be simultaneous 2.
Terminal Ileal Involvement: The Critical Distinguishing Feature
The terminal ileal ulcers are the key finding that narrows the differential:
Terminal ileal involvement is common in AS patients with gut inflammation 3. In Korean AS patients undergoing ileocolonoscopy, inflammatory lesions were "found often in the terminal ileum," with 40 of 108 patients showing inflammatory lesions 3.
Terminal ileitis in AS patients represents a spectrum: Among 40 AS patients with inflammatory lesions, 23 had subclinical gut inflammation associated with AS, while 12 were ultimately diagnosed with Crohn's disease, 4 with intestinal tuberculosis, and 1 with ulcerative colitis 3.
Ulcerative colitis classically does not involve the terminal ileum except in rare cases of "backwash ileitis" in extensive colitis 4, making answer (a) unlikely as a standalone diagnosis.
Why Not Crohn's Disease Alone (Answer E)?
While Crohn's disease is certainly possible and remains a strong differential, the question asks for the most likely diagnosis BEFORE biopsy results in a patient with established AS. The clinical context of pre-existing spondyloarthritis makes "IBD-associated SpA" the more encompassing and accurate pre-biopsy diagnosis.
Crohn's disease would be a subset within the broader category of IBD-associated SpA 2. The 2024 guidelines emphasize that "the diagnosis of IBD and the distinction between Crohn's disease, ulcerative colitis and unclassified IBD should rely on commonly accepted criteria as defined by ECCO" 2—which requires histology.
Excluding Other Options
Celiac disease (Answer C): Does not present with bloody stools or ulcers in the terminal ileum. This is a non-starter.
All of the above (Answer D): Illogical, as UC doesn't typically cause terminal ileal ulcers, and celiac disease doesn't fit the presentation.
Clinical Pitfalls and Red Flags
Don't Miss Infectious Mimics
Critical caveat: In AS patients with new gastrointestinal symptoms and terminal ileal ulcers, intestinal tuberculosis must be excluded, especially in endemic regions 5. In the Korean study, 4 of 40 AS patients with inflammatory lesions had intestinal tuberculosis 3. The 2026 case report emphasizes that ITB can mimic Crohn's disease with concomitant axSpA, and premature immunosuppression can be catastrophic 5.
Before finalizing the diagnosis, ensure:
- Stool cultures and TB testing are negative
- Histology shows granulomas WITHOUT caseating necrosis
- No acid-fast bacilli on special stains
Screening Recommendations
The 2024 consensus identifies specific "red flags" for IBD screening in SpA patients 2:
- Major criteria (one sufficient): Chronic diarrhea >4 weeks, rectal bleeding, nocturnal diarrhea, perianal disease
- Minor criteria (≥3 needed): Abdominal pain, weight loss, anemia, elevated inflammatory markers
This patient has multiple major criteria (bloody stools, abdominal pain), warranting immediate gastroenterology referral 2, 6.
Disease Activity Correlation
Research shows that abdominal pain in AS patients correlates with higher disease activity scores and acute ileal inflammation 6. The presence of symptoms requiring colonoscopy (versus routine screening) increases the odds of finding inflammatory lesions nearly 4-fold (OR 3.96) 3.
Practical Management Approach
While awaiting biopsy:
- Exclude infections first: TB testing, stool cultures, C. difficile
- Assess SpA disease activity: Use ASDAS-CRP (recommended by 2024 guidelines for axial disease) 2
- Check faecal calprotectin: Elevated levels support IBD diagnosis, though interpret cautiously as AS patients can have subclinical inflammation 2
- Obtain segmental biopsies: From terminal ileum and multiple colonic sites to establish extent and histologic pattern 4
Treatment implications: If IBD is confirmed, anti-TNF agents (particularly infliximab) are first-line therapy for both active IBD and axial SpA 2, 1. This is crucial because NSAIDs—typically used for AS—can exacerbate IBD 1.