Q5 LONG ANSWER 10M ★ High-yield

Describe inflammatory bowel disease. Differentiate Crohn disease from ulcerative colitis.

Pathology · 2nd Professional — Para-Clinical

Definition

Inflammatory bowel disease comprises CROHN disease and ulcerative colitis, chronic relapsing inflammatory disorders resulting from an inappropriate mucosal immune response to normal gut flora in a genetically susceptible person.

Pathogenesis

Genetic susceptibility — NOD2 (CARD15) mutations in Crohn disease impair the innate response to bacterial peptidoglycan; HLA associations; familial clustering
Mucosal immune dysregulation — excessive TH1 and TH17 response in Crohn disease; a more TH2-like response in ulcerative colitis
Defective epithelial barrier allowing luminal antigens access
Gut microbiota — disease does not occur in germ-free animals, implicating the flora rather than a specific pathogen
Environmental — smoking is protective in ulcerative colitis but worsens Crohn disease, one of the more striking contrasts in medicine; appendicectomy is also protective in ulcerative colitis

Crohn Disease and Ulcerative Colitis Compared

Feature	CROHN disease	Ulcerative colitis
Site	Any part of the gut, mouth to anus; terminal ileum most often; rectum often spared	Colon only; always involves the rectum and extends proximally in continuity
Distribution	Skip lesions — discontinuous	Continuous
Depth	Transmural	Mucosa and submucosa only
Granulomas	Present in about 50% — non-caseating	Absent
Naked eye	"cobblestone" mucosa; deep serpiginous and linear ulcers; "creeping fat"; thickened rubbery wall with a narrowed "string sign" lumen	Broad shallow ulcers; pseudopolyps (islands of regenerating mucosa); a shortened, featureless "lead-pipe" colon with loss of haustra
Microscopy	Transmural lymphoid aggregates, non-caseating granulomas, fissuring ulcers, submucosal fibrosis, neuronal hyperplasia	Crypt abscesses, crypt distortion and branching, goblet cell depletion, basal plasmacytosis; inflammation stops at the muscularis mucosae
Fistulae and strictures	Common — a consequence of transmural disease	Rare
Perianal disease	Common — fissures, fistulae, abscesses, skin tags	Uncommon
Toxic megacolon	Uncommon	Characteristic and dangerous
Malignant risk	Increased, but less	Markedly increased — especially with pancolitis of over 10 years
Smoking	Worsens it	Protective
Surgery	Not curative — recurs at the anastomosis	Curative — colectomy removes the disease

CLINICAL PEARL

Almost every difference follows from depth and distribution. Crohn disease is transmural, so it produces fistulae, abscesses, strictures and serosal fat wrapping; and it is patchy and can affect any level, so surgery cannot cure it. Ulcerative colitis is mucosal and confined to the colon in continuity, so it bleeds freely but rarely fistulates — and removing the colon removes the disease. Derive the table from those two facts rather than memorising it.

Clinical Features

Feature	CROHN disease	Ulcerative colitis
Diarrhoea	Often without blood	Bloody diarrhoea with mucus — the hallmark
Pain	Right iliac fossa, colicky; may mimic appendicitis	Left-sided, with tenesmus and urgency
Mass	Palpable right iliac fossa mass	Absent
Malabsorption	Common — B12 (terminal ileum), bile salts, fat; gallstones and oxalate renal stones	Uncommon
Systemic upset	Weight loss, fever, growth failure in children	Less prominent except in severe attacks

Extra-intestinal manifestations occur in both — erythema nodosum, pyoderma gangrenosum, anterior uveitis, seronegative arthritis, ankylosing spondylitis, primary sclerosing cholangitis (strongly linked to ulcerative colitis), and secondary AA amyloidosis

Complications

Crohn disease — strictures with obstruction, fistulae (enterocutaneous, enterovesical, enteroenteric), abscess, perforation, perianal disease, malabsorption, short bowel syndrome after repeated resections
Ulcerative colitis — severe haemorrhage, toxic megacolon (a dilated, thin-walled, atonic colon at risk of perforation), perforation, and colorectal carcinoma
Cancer surveillance in ulcerative colitis — risk rises with duration (from about 8 to 10 years) and extent; colonoscopic surveillance with biopsies looking for dysplasia, which is the marker for colectomy

Intestinal Tuberculosis — the Indian Differential

Feature	Intestinal tuberculosis	CROHN disease
Granulomas	Caseating; large and confluent; numerous	Non-caseating; small, sparse, discrete
Ulcers	Transverse (circumferential), following lymphatics	Longitudinal and serpiginous; cobblestoning
Site	Ileocaecal region; the caecum is contracted and pulled up	Terminal ileum; any level; skip lesions
Lymph nodes	Enlarged, often with central necrosis on CT	Reactive only
Special stains and tests	Ziehl–Neelsen positive; CBNAAT and culture positive	Negative
Perianal disease	Uncommon	Common
Response to antituberculous therapy	Improves	No improvement

Where the distinction cannot be made, a therapeutic trial of antituberculous therapy is often given first, since the consequence of missing tuberculosis and giving immunosuppression is far worse than a short delay

Applied Aspects

In India, intestinal tuberculosis is the crucial differential of Crohn disease — both affect the ileocaecal region with granulomas. Tuberculosis gives caseating, larger, confluent granulomas, transverse ulcers, and a positive Ziehl–Neelsen stain or CBNAAT; Crohn gives non-caseating, small, sparse granulomas and longitudinal ulcers. Giving steroids for misdiagnosed tuberculosis is dangerous
Colonoscopy with multiple biopsies is the investigation of choice; the pattern of involvement matters as much as the histology
Faecal calprotectin distinguishes inflammatory from functional bowel disease non-invasively and is useful for monitoring
Treatment — 5-aminosalicylates (better in ulcerative colitis), corticosteroids for flares, azathioprine and methotrexate for maintenance, and anti-TNF and other biological agents in resistant disease
Screen for latent tuberculosis before starting anti-TNF therapy — essential in India, since these agents dissolve the granulomas containing the organism
Nutritional support and psychological care matter greatly; these are lifelong diseases of young people, and quality of life is a legitimate treatment goal
Toxic megacolon is a surgical emergency — a transverse colon over 6 cm with systemic toxicity; abdominal radiographs are repeated daily during a severe attack and colectomy performed if it fails to settle
Incidence is rising in India, particularly of ulcerative colitis in urban populations, so the disease can no longer be regarded as uncommon here
Infective colitis must be excluded at every flare — amoebiasis, Clostridioides difficile and bacterial dysentery all mimic a relapse and are made worse by immunosuppression
Bone protection, vaccination and skin cancer awareness are part of long-term care, given the prolonged steroid and immunosuppressive exposure

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