Your next patient, 26, has diarrhoea and weight loss for 3 months and wants your thoughts. Here's how the Crohn's disease case unfolds and what to pick up on the way.

Spotting a crohn's disease case can be easy to miss when a 26-year-old reports months of diarrhoea and weight loss that sound like IBS or a lingering infection. The traps in the history are focusing on frequency while overlooking blood or mucus, crampy right lower pain, mouth ulcers, and perianal soreness or lumps, especially in a smoker.

This page uses the scenario of your next patient, 26, who says, "Diarrhoea and weight loss over the last few months." It walks through how the case presents, what to pick up in the history, the tests that help separate inflammatory bowel disease from IBS such as faecal calprotectin with colonoscopy to follow, and the teaching points on distribution, transmural inflammation, perianal disease and how this differs from ulcerative colitis, while remembering to exclude infection with stool culture.

How the case presents

Your next patient, 26, has diarrhoea and weight loss for 3 months and wants your thoughts. In their own words: "Diarrhoea and weight loss over the last few months."

Observations on arrival:

Observation	On arrival
Heart rate	90 beats per minute
Blood pressure	118/74 mmHg
Respiratory rate	16 breaths per minute
Oxygen saturation	99%
Temperature	37.6 °C

What to pick up in the history

These are the findings the case is built around. Each one moves you towards or away from Crohn's disease, and the scorecard checks whether you found it.

Months of loose stools, sometimes blood or mucus: chronic diarrhoea over months separates inflammatory bowel disease from infective and functional causes.
Crampy pain in the right lower abdomen: right lower quadrant pain points to terminal ileal disease, typical of Crohn's.
Unintentional weight loss with fatigue: weight loss marks organic inflammatory disease rather than irritable bowel syndrome.
Recurrent painful mouth ulcers: oral ulceration reflects the gut-wide involvement characteristic of Crohn's disease.
Perianal soreness, skin tags and lumps: perianal disease is highly specific for Crohn's and argues against ulcerative colitis.
Current smoker: smoking worsens Crohn's while protecting against ulcerative colitis, shifting the diagnosis.
Colicky pain with bloating and vomiting: suggests a stricture with subacute obstruction, a transmural complication of Crohn's.
A relative has inflammatory bowel disease: family history substantially raises the pre-test probability of inflammatory bowel disease.

Red flags you must not miss

The case marks these as must-elicit. They only come out if you ask.

Colicky pain with bloating and vomiting.
He has felt more and more run-down and unwell with the ongoing symptoms.

Investigations and what they showed

Faecal calprotectin: Markedly raised. Why it helps: A sensitive non-invasive marker of bowel inflammation that distinguishes inflammatory bowel disease from irritable bowel syndrome and flags the need for colonoscopy.
Stool culture: Negative for pathogens (including Clostridioides difficile toxin).
C-reactive protein: Raised.
Full blood count: Anaemia (iron/B12 deficiency or chronic disease) with a raised platelet count. Why it helps: anaemia and thrombocytosis support chronic inflammation and nutritional deficiency; guides replacement.
Vitamin B12: Low (terminal ileal Crohn's impairs B12 absorption).
Histology / biopsy: Transmural chronic inflammation with non-caseating granulomas and skip lesions on biopsies taken at colonoscopy. Why it helps: histology confirms Crohn's disease (granulomas, transmural pattern) and distinguishes it from ulcerative colitis and infection.
Colonoscopy: Patchy inflammation with skip lesions, deep serpiginous ulcers and a cobblestone mucosa (often with terminal-ileal involvement and rectal sparing); biopsies for histology. Why it helps: the key diagnostic test, it shows the discontinuous, deep-ulcerating, cobblestone pattern of Crohn's (vs the continuous mucosal disease of UC) and provides biopsies.
MRI small bowel enterography: Small-bowel wall thickening/enhancement, skip lesions, strictures and fistulae.
Inspect the perianal area and anus: Fleshy, oedematous skin tags at the anal verge with surrounding induration and discolouration. Why it helps: the most characteristic external sign of Crohn's disease and the one most often missed because nobody looks.
Inspect the mouth and oral cavity: A shallow ulcer with a yellow-grey base and a red halo on the buccal mucosa. Why it helps: crohn's disease can affect anywhere from mouth to anus, and recurrent aphthous ulceration is its oral manifestation. In a young man with chronic diarrhoea and weight loss, mouth ulcers plus perianal disease is close to diagnostic and separates Crohn's from ulcerative colitis and from IBS.
Inspect the calves and shins: Tender, raised, dusky red-purple nodules over the front of the shin, bruise-like and ill-defined. Why it helps: erythema nodosum is the commonest cutaneous extra-intestinal manifestation of IBD and tracks with disease activity, it typically settles as the bowel inflammation is controlled. Alongside his sore red eyes and achy joints it confirms this is a multisystem inflammatory disease, not simple infective diarrhoea.
Inspect the fingernails: Bulbous drumstick fingertips with loss of the normal angle between nail fold and nail. Why it helps: clubbing occurs in chronic active Crohn's disease and is a quiet marker of how long and how active the inflammation has been. It is one of the few signs that argue strongly against IBS in a young person with diarrhoea, and it belongs alongside faecal calprotectin in the decision to proceed to colonoscopy.
Inspect the patient's hydration and nutritional state: A thin, tired-looking 26-year-old with hollowed temples, prominent clavicles and clothes that are visibly too big for him.
Inspect the eyes: Look for a localised wedge of redness on the white of the eye (episcleritis) or a dull red circumcorneal flush with photophobia (anterior uveitis). Why it helps: he describes intermittent sore red eyes. Episcleritis mirrors bowel activity, whereas uveitis runs independently of it and threatens sight, uveitis needs a same-day ophthalmology referral, not reassurance, so distinguishing the two at the bedside genuinely changes what happens next.
Inspect the abdomen: The abdomen is flat and soft with no distension at rest, but he holds the right iliac fossa and describes it filling and cramping after meals. Why it helps: terminal ileal disease localises to the right iliac fossa and mimics appendicitis. His episodes of colicky pain with bloating and vomiting are the warning of an evolving stricture, the reason to request MRI small-bowel enterography rather than relying on colonoscopy alone.

Imaging findings

CT enterography: Bowel wall thickening, strictures, fistulae or an abscess/collection.
X-ray chest: Normal chest x-ray clear lungs no consolidation.

Why the diagnosis is Crohn's disease

The decisive diagnosis is crohn's disease, chronic transmural inflammation that can affect any part of the gut from mouth to anus, here with chronic diarrhoea, right-lower-quadrant pain, weight loss, mouth ulcers, perianal disease and extra-intestinal features, in a young smoker. 'Inflammatory bowel disease' is the correct umbrella, but distinguishing crohn's from ulcerative colitis matters: Crohn's is patchy/skip, transmural, granulomatous, mouth-to-anus (often terminal ileum), with strictures, fistulae and perianal disease, and is associated with smoking; UC is continuous mucosal disease from the rectum with bloody diarrhoea, associated with PSC and toxic megacolon, and is (paradoxically) commoner in non-/ex-smokers. Work up with faecal calprotectin, bloods and, after excluding infection (stool culture/C. difficile), colonoscopy with biopsies, plus MRI small-bowel enterography for small-bowel/complication assessment.

Crohn's disease at a glance

What it is

Crohn's disease is a chronic inflammatory bowel disease that can affect any part of the gastrointestinal tract, mouth to anus, though it most often involves the terminal ileum and colon. Unlike ulcerative colitis, inflammation is transmural (full thickness of the bowel wall) and patchy, with normal "skip areas" between diseased segments.

Pathogenesis

An abnormal immune response to gut bacteria develops in a genetically susceptible person, driven by triggers such as smoking. Chronic transmural inflammation forms non-caseating granulomas and, over time, strictures (fibrosis) and fistulae (inflammation tracking through the full wall to adjacent structures or skin), complications that distinguish Crohn's from the mucosa-only disease of ulcerative colitis.

Who it affects

Crohn's typically presents in young people, peaking between 15 and 30. It affects roughly 1 in 350 people in the UK, more common with an affected first-degree relative, in smokers, and in people of European, especially Ashkenazi Jewish, descent.

How serious it is

Crohn's is a lifelong, relapsing-remitting condition. Untreated disease causes strictures, fistulae, abscesses and malnutrition, and carries a raised long-term colorectal cancer risk.

Causes and risk factors

The exact cause is unknown, but disease results from an abnormal immune response to gut bacteria in a genetically susceptible person. Smoking is the strongest modifiable risk factor and worsens the course (the opposite of ulcerative colitis, where it is protective). Family history, a Western diet, and prior GI infection also contribute.

Investigations and lab tests

Faecal calprotectin is a useful non-invasive first test that separates IBD from irritable bowel syndrome and prompts further work-up. Colonoscopy with biopsies is the key diagnostic test, showing patchy inflammation, skip lesions and non-caseating granulomas on histology. MRI small-bowel enterography assesses small-bowel disease, strictures and fistulae without radiation. Bloods assess activity and nutritional deficiency.

At a glance: First: faecal calprotectin. Diagnostic: colonoscopy + biopsies. Small-bowel disease: MRI enterography.

Teaching points

Crohn's disease is chronic transmural inflammation that can affect any part of the gut (mouth to anus, often the terminal ileum), suspect it from chronic diarrhoea, abdominal pain, weight loss, mouth ulcers, perianal disease and extra-intestinal features in a young person.
Distinguish crohn's (skip lesions, transmural, granulomas, strictures/fistulae, perianal disease, smoking-associated) from ulcerative colitis (continuous mucosal disease from the rectum, bloody diarrhoea, PSC/toxic megacolon, smoking-protective).
Faecal calprotectin is a useful non-invasive marker of bowel inflammation that separates IBD from IBS and prompts colonoscopy.
Exclude infection (stool culture, C.
Colonoscopy with biopsies is the key diagnostic test; MRI small-bowel enterography assesses small-bowel disease and complications without radiation.
Advise smoking cessation (smoking worsens Crohn's), correct nutritional deficiencies (iron, B12, vitamin D), and consider VTE prophylaxis in flares.
Arrange colorectal-cancer surveillance and multidisciplinary IBD care for this lifelong relapsing-remitting condition.

Practise a similar case

Reading a case is one thing, but catching the clue of recurrent mouth ulcers while you steer a history under a little pressure is different. Try another gastro case where you set the pace, decide what to clarify, and keep rapport as the minutes tick by. Take a history from the next patient below.

Here's one from the same specialty, with the diagnosis left for you to find. Your next patient, a 55-year-old man, has noticed his tummy swelling for 3 weeks. Take this patient's history in Diagnosica. It's free to start, no card needed.

Crohn's disease case presentation: history, findings and tests