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Inflammatory Bowel Disease — Crohn Disease

Transmural inflammatory bowel disease with skip lesions that can affect any GI segment from mouth to anus, classically involving terminal ileum and causing fistulas, strictures, and malabsorption

Review date
reviewed 2026-05-04
Reading time
9 min read
Author
Dr Kola Tytler MBBS CertHE MBA MSt MRCGP
Gastroenterology & Hepatologycommonlong-term-condition
On this page
Condition details
Gastroenterology & Hepatology
common
9 min read
reviewed 2026-05-04
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About This Page

This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.

Key points

  • Crohn = transmural inflammation, skip lesions, any GI location, terminal ileum common
  • Complications include fistulas, abscesses, strictures, SBO, malabsorption, kidney stones, and gallstones
  • Diagnosis: colonoscopy with ileoscopy and biopsy; enterography defines small bowel and penetrating disease
  • Steroids induce remission; biologics/immunomodulators maintain remission
  • Smoking worsens Crohn and increases recurrence after surgery

Overview

Crohn disease is a chronic relapsing IBD with transmural inflammation and discontinuous skip lesions. It can involve the entire GI tract but most often affects the terminal ileum and colon. Transmural disease produces strictures, fistulas, abscesses, and perianal disease.

Epidemiology

IBD prevalence is increasing in North America. Crohn often presents in adolescence or early adulthood. Risk factors include family history, Ashkenazi Jewish ancestry, smoking, and dysregulated immune responses to intestinal microbiota.

Clinical Features

Symptoms
Chronic diarrhea with abdominal pain
Weight loss, fatigue, anorexia
Extraintestinal manifestations: arthritis, erythema nodosum, episcleritis, uveitis
Fever, obstruction, severe bleeding, abscess, or toxic megacolon features
Signs
Abdominal tenderness
Pallor or weight loss
Perianal disease favors Crohn; rectal bleeding/tenesmus favors UC
Peritonitis, sepsis, or marked distension

Investigations

First-line
CBC, CMP, CRP/ESRAnemia, leukocytosis, hypoalbuminemia, inflammatory burden
Stool studies including C difficileExclude infection before immunosuppression
Fecal calprotectinSupports intestinal inflammation and helps distinguish IBD from IBS
Second-line
Colonoscopy with biopsyDefines extent, severity, and histology
CT or MR enterographySmall bowel disease, strictures, fistulas, abscesses; especially Crohn
Specialist
Pelvic MRIComplex perianal Crohn disease
Flexible sigmoidoscopyAcute severe colitis when full colonoscopy is unsafe

Management

ACG Clinical Guideline: Management of Crohn Disease in Adults
1
Induction
  • Budesonide for mild ileocecal Crohn; 5-ASA for mild UC
  • Systemic corticosteroids for moderate-severe flares
  • Hospitalize severe colitis, obstruction, abscess, or systemic toxicity
2
Maintenance
  • Steroids are never maintenance therapy
  • Use immunomodulators, biologics, or targeted small molecules according to severity
  • Vaccinate and screen for TB/HBV before biologics
3
Surgery/surveillance
  • Crohn surgery treats complications but is not curative
  • Colectomy cures UC colitis and is indicated for dysplasia/cancer or toxic megacolon
  • Colon cancer surveillance after long-standing colitis

Complications

  • Crohn: Fistulas, abscesses, strictures, SBO, malabsorption.
  • UC: Toxic megacolon, severe bleeding, PSC, colorectal cancer.
  • Extraintestinal disease: Arthritis, uveitis, erythema nodosum, pyoderma gangrenosum.
  • Medication complications: Infection and malignancy risks with immunosuppression.
USMLE Step 2 CK Exam Tips
  • 1Crohn = skip lesions + transmural inflammation + fistulas; UC = continuous mucosal disease starting at rectum
  • 2Perianal fistula strongly favors Crohn
  • 3UC toxic megacolon: fever, tachycardia, abdominal distension, colon >6 cm
  • 4Steroids induce remission but never maintain remission
  • 5PSC + UC = high CRC risk
  • 6Terminal ileum Crohn causes B12 deficiency, bile acid diarrhea, gallstones, calcium oxalate stones
  • 7Exclude C difficile before treating an IBD flare

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Verified Sources & References

ACG Clinical Guideline: Crohn Disease in Adults
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