Bone Wizardry — GI
Crohn's vs UC
The two faces of IBD. One burrows deep. The other stays on the surface and bleeds. Knowing which does what is the entire game on boards.
The Trap
A 28-year-old woman presents with chronic bloody diarrhea, abdominal pain, and a 10-lb weight loss. Colonoscopy shows continuous inflammation from the rectum to the splenic flexure with loss of haustra and pseudopolyps. Biopsy shows crypt abscesses limited to the mucosa. Which feature MOST strongly distinguishes this from Crohn's disease?
Good catch. Continuous inflammation starting at the rectum is the single strongest distinguisher. Crohn's is patchy (skip lesions) and can spare the rectum entirely. UC always starts at the rectum and extends proximally without gaps. Bloody diarrhea, weight loss, and pseudopolyps can appear in both.
Close, but not quite. That finding can appear in both Crohn's and UC. The key distinguisher here is continuous inflammation starting at the rectum — UC always begins at the rectum and extends proximally without skip areas. Crohn's is patchy and can spare the rectum. That pattern of involvement is the most reliable differentiator on boards.
Side by Side
The Big Picture
Everything that makes them different, in one glance.
CROHN'S
LocationMouth to anus (anywhere in GI tract). Terminal ileum is most common.
PatternSkip lesions — patchy, discontinuous. Can spare the rectum.
DepthTransmural — full thickness, all layers of bowel wall.
Gross appearanceCobblestoning, creeping fat, bowel wall thickening, strictures.
HistologyNon-caseating granulomas (30-60% of cases). Transmural inflammation.
StoolOften non-bloody. Watery or fatty (malabsorption).
ComplicationsFistulas, strictures, abscesses. String sign on barium swallow.
Cancer riskLower than UC (still elevated).
SmokingMakes it WORSE.
SurgeryNot curative. Recurrence is common.
Serologic markerASCA+ (anti-Saccharomyces cerevisiae)
UC
LocationColon only. Always starts at the rectum, extends proximally.
PatternContinuous — no skip lesions. No gaps in inflammation.
DepthMucosal/submucosal only — superficial. Does NOT penetrate full thickness.
Gross appearancePseudopolyps, loss of haustra ("lead pipe"), friable mucosa.
HistologyCrypt abscesses. No granulomas. Mucosal ulceration only.
StoolBloody diarrhea — the hallmark. Often with mucus.
ComplicationsToxic megacolon, perforation. No fistulas or strictures.
Cancer riskHigher. Increases with duration and extent of disease.
SmokingProtective. (Not a treatment recommendation.)
SurgeryCurative (total colectomy removes the disease).
Serologic markerp-ANCA+ (perinuclear antineutrophil cytoplasmic)
Memory Hook
Tap to reveal
"Crohn's CRAWLS deep" — Cobblestoning, cReeping fat, skAnywhere (mouth to anus), Wall (transmural), skip Lesions, Strictures/fistulas.
Memory Hook
Tap to reveal
"UC is a SURFACE wound" — Superficial (mucosal only), Uninterrupted (continuous), Rectum always, Friable + pseudopolyps, Always bloody, Crypt abscesses, curable with colEctomy.
Sort It Out
Feature Sorting
Drag each feature into the right disease. Wrong guesses bounce back.
CROHN'S
UC
Beyond the Gut
Extraintestinal Manifestations
Both can cause problems outside the GI tract. Some track with disease activity. Some don't.
Joints
Which gets arthritis?
Both. Peripheral arthritis tracks with disease activity (flares with gut). Ankylosing spondylitis does NOT track — it runs its own course.
Skin
Erythema nodosum vs Pyoderma gangrenosum?
Erythema nodosum = more common in Crohn's. Tracks with disease. Pyoderma gangrenosum = more common in UC. Does NOT track reliably.
Eyes
What eye problems?
Uveitis/episcleritis — both diseases. Tracks with disease activity. Red eye + IBD flare = think extraintestinal.
Liver
Which causes PSC?
Primary Sclerosing Cholangitis = strongly associated with UC (70% of PSC patients have UC). Does NOT track with disease — can progress even after colectomy. Increases cholangiocarcinoma risk.
Nutrition
Which causes malabsorption?
Crohn's — terminal ileum involvement impairs B12 and bile salt absorption. Also fat-soluble vitamin deficiency (A, D, E, K). UC doesn't usually cause malabsorption because the colon mainly absorbs water.
Stones
Gallstones or kidney stones?
Crohn's gets both. Gallstones: impaired bile salt reabsorption in terminal ileum. Oxalate kidney stones: unabsorbed fatty acids bind calcium, leaving free oxalate to absorb and crystallize.
| Finding | Crohn's | UC |
|---|---|---|
| Peripheral arthritis | Yes (tracks) | Yes (tracks) |
| Ankylosing spondylitis | Yes | Yes |
| Erythema nodosum | More common | Yes |
| Pyoderma gangrenosum | Yes | More common |
| Uveitis / Episcleritis | Yes | Yes |
| Primary Sclerosing Cholangitis | Rare | Strong association |
| Malabsorption (B12, fat-sol) | Yes (terminal ileum) | No |
| Gallstones | Yes (bile salt loss) | No |
| Oxalate kidney stones | Yes (fat malabsorption) | No |
Work It Out
Diagnostic Decision Tree
A patient presents with chronic diarrhea and abdominal pain. Walk through it.
Is the diarrhea bloody?
Elimination Game
Clinical Vignettes
Each clue eliminates one option. By the end, only the answer remains.
Test Yourself
Quiz
4 random questions from a pool of 11. Shuffled answers. No timer.