About This Page
This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Crohn's can affect ANY part of the GI tract — most commonly the terminal ileum and colon
- Transmural inflammation with skip lesions (unlike UC which is continuous and mucosal only)
- Histology: non-caseating granulomata (pathognomonic but present in only ~60%)
- Induce remission: steroids (prednisolone or budesonide for ileal disease). Maintain remission: azathioprine/mercaptopurine or methotrexate
- Biologics (infliximab, adalimumab) for moderate-severe or steroid-refractory/dependent disease
- Surgery is NOT curative (unlike UC) — reserve for complications (strictures, fistulae, perforation)
Overview
Crohn's disease is a chronic relapsing-remitting inflammatory bowel disease characterised by transmural granulomatous inflammation that can affect any segment of the GI tract from mouth to anus. The terminal ileum is the most commonly affected site (~40%), followed by ileocolonic and colonic disease. Characteristic features include skip lesions (inflamed segments interspersed with normal mucosa), cobblestoning of the mucosa, fissuring ulcers, non-caseating granulomata, and a tendency to form strictures, fistulae, and abscesses.
Epidemiology
Crohn's disease has a UK prevalence of approximately 150 per 100,000. Bimodal age of onset with peaks at 15–30 and 60–80 years. More common in females and in Northern European/Ashkenazi Jewish populations. Smoking is the strongest modifiable risk factor and doubles the risk (unlike UC where smoking is protective). Family history confers a 5–20× increased risk. NOD2/CARD15 gene variants are the strongest genetic association.
Clinical Features
Symptoms
Chronic diarrhoea (usually non-bloody, unlike UC)
Abdominal pain — often right iliac fossa (terminal ileum)
Weight loss and malnutrition
Fatigue
Perianal symptoms: pain, discharge, fistulae, skin tags
Oral ulceration
Growth failure in children
Acute abdomen (perforation, obstruction)
Signs
RIF tenderness or mass (terminal ileal inflammation or abscess)
Perianal fistulae, abscess, or skin tags
Oral aphthous ulcers
Signs of malnutrition: low BMI, pallor, angular stomatitis
Extraintestinal manifestations: erythema nodosum, pyoderma gangrenosum, uveitis, arthritis, clubbing
Signs of intestinal obstruction: distension, high-pitched bowel sounds
Investigations
First-line
BloodsFBC (anaemia, raised WCC/platelets), CRP/ESR (raised), albumin (low in malnutrition), U&Es, LFTs
Faecal calprotectinElevated (>250 µg/g strongly suggestive of IBD). Useful to distinguish IBD from IBS
Stool MC&SExclude infective diarrhoea including C. difficile
Second-line
Ileocolonoscopy with biopsiesGold standard for diagnosis. Macroscopy: skip lesions, deep ulceration, cobblestoning. Histology: transmural inflammation, non-caseating granulomata
MRI small bowel (MR enterography)Assess small bowel disease extent, strictures, fistulae, and abscess — avoids radiation
Specialist
CT abdomen/pelvisAcute presentations — assess for perforation, abscess, obstruction
Capsule endoscopyIf small bowel disease suspected but MRI and colonoscopy non-diagnostic
EUA + MRI pelvisPerianal disease assessment — map fistula tracts before surgical planning
1
Inducing remission
- First-line: oral prednisolone 40 mg/day tapered over 8 weeks
- Ileal/ileocaecal disease: budesonide 9 mg/day (fewer systemic side effects) as alternative
- Severe/refractory: IV hydrocortisone, then consider biological therapy
2
Maintaining remission
- First-line: azathioprine 2–2.5 mg/kg/day or mercaptopurine 1–1.5 mg/kg/day
- Check TPMT genotype before starting (risk of myelosuppression)
- Alternative: methotrexate SC/IM (if thiopurine intolerant/failed)
- Steroids must NOT be used for maintenance — unacceptable side effect profile
3
Biologic therapy
- Anti-TNF: infliximab (IV) or adalimumab (SC) — for moderate-severe, steroid-dependent, or fistulating disease
- Anti-integrin: vedolizumab — gut-selective, for anti-TNF failure
- Anti-IL-12/23: ustekinumab — for anti-TNF failure
- Combination therapy (infliximab + azathioprine) may be more effective than monotherapy
4
Perianal disease
- Abscess: incision and drainage + antibiotics (metronidazole ± ciprofloxacin)
- Simple fistula: seton insertion
- Complex fistula: anti-TNF therapy (infliximab), examination under anaesthesia, long-term seton
5
Surgery
- NOT curative — aim to preserve bowel length
- Indications: stricture, obstruction, perforation, abscess, fistula, failure of medical therapy
- Common procedures: limited ileal resection, stricturoplasty
- Post-operative recurrence: ~50% at 10 years at the neo-terminal ileum
Complications
- Strictures: Chronic inflammation → fibrosis → bowel obstruction — commonest surgical indication
- Fistulae: Transmural disease → enteroenteric, enterocutaneous, enterovesical, rectovaginal, perianal tracts
- Abscess: Intra-abdominal or perianal — requires drainage and antibiotics
- Malnutrition: B12 deficiency (terminal ileal disease), iron deficiency, vitamin D deficiency, low albumin
- Colorectal cancer: Increased risk with extensive colonic Crohn's — surveillance colonoscopy
- Osteoporosis: From steroid use and malabsorption
- Thromboembolic disease: IBD is an independent risk factor for VTE — especially during flares
UKMLA Exam Tips
- 1Crohn's = skip lesions + transmural + granulomata + mouth-to-anus. UC = continuous + mucosal only + starts at rectum + pseudopolyps
- 2Smoking WORSENS Crohn's but PROTECTS against UC — classic exam differentiator
- 3Faecal calprotectin distinguishes IBD from IBS — this is the first-line test in primary care for chronic diarrhoea
- 4Check TPMT before starting azathioprine — 1 in 300 are homozygous deficient (severe myelosuppression)
- 5RIF mass + weight loss + chronic diarrhoea in a young person = think Crohn's (can mimic appendicitis or TB)
- 6Steroids induce remission but must NEVER be used for maintenance — this is a frequently tested principle
practicetest your knowledge on crohn's diseaseApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — gastroenterology and beyond.
open q-bank