Crohn's disease

Crohn's disease is one of the two main inflammatory bowel diseases (the other is ulcerative colitis). Unlike UC, which stays in the colon mucosa, Crohn's involves the full thickness of the bowel wall (transmural) and can strike anywhere from mouth to anus — most often the terminal ileum and right colon — in a 'skip lesion' pattern. The pathogenesis is dysregulated immune response to gut microbiota in genetically susceptible people; smoking, Western diet, NSAID use, and antibiotics in early life raise risk. Peak onset is 15–35 years, with a smaller second peak at 60+. Complications include fistulae (bowel-to-skin, bowel-to-bladder), strictures with obstruction, abscesses, malabsorption (B12, fat-soluble vitamins, iron), growth failure in children, and extra-intestinal manifestations (joint pain, eye inflammation, skin lesions, primary sclerosing cholangitis).

• CRP, ESR — disease activity markers; elevated in flares, often normal-ish in stable disease.
• Faecal calprotectin — far more sensitive than CRP for active bowel inflammation; >250 µg/g strongly suggests active IBD.
• CBC — anaemia (iron, B12, folate, anaemia of chronic disease — often combined).
• Ferritin, iron studies, B12, folate — malabsorption screen.
• Albumin — low in active disease + malabsorption + protein-losing enteropathy.
• Vitamin D, calcium — low; affect bone health.
• LFTs — elevated suggest PSC (primary sclerosing cholangitis), a known Crohn's complication.
• Stool studies — exclude C. difficile, parasites, infectious colitis.
• Colonoscopy + biopsy — diagnostic standard.

• Chronic diarrhoea (often non-bloody, unlike UC)
• Crampy abdominal pain, classically right lower quadrant
• Unintentional weight loss
• Fatigue (anaemia + inflammation)
• Fever during flares
• Mouth ulcers, perianal disease (fistulae, fissures, abscesses)
• Extra-intestinal: arthritis, uveitis, erythema nodosum, pyoderma gangrenosum