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.
Before We Start
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.
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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.
Smoking paradox: Smoking worsens Crohn's but is protective against UC. This is one of the most tested board facts in IBD. The mechanism likely involves nicotine's effects on mucus production, colonic blood flow, and immune modulation.
Bloody diarrhea does NOT automatically mean UC. Crohn's colitis can also cause bloody stool. The distinguishing factor is the pattern of inflammation (continuous vs skip) and depth (mucosal vs transmural), not the presence of blood.
"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.
PSC + UC = high-yield combo. 70% of PSC patients have UC. PSC does NOT improve after colectomy and independently increases cholangiocarcinoma risk. If a board question gives you a UC patient with elevated alk phos and bile duct beading on MRCP, that's PSC.
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?
Bloody diarrhea. Where is the inflammation on colonoscopy?
Non-bloody diarrhea. Where does imaging show the disease?
Continuous rectal inflammation. What does biopsy show?
Ulcerative Colitis. Continuous inflammation from rectum + mucosal-only + crypt abscesses = classic UC. Watch for toxic megacolon and increased colorectal cancer risk with duration.
That's unusual. Continuous rectal inflammation with granulomas is atypical. Consider Crohn's colitis (Crohn's limited to the colon) — it CAN look continuous in some cases, and granulomas are the histologic giveaway. This is a board trap: location suggests UC, but histology wins.
Patchy inflammation with possible skip lesions. What's the histology?
Crohn's Disease. Skip lesions + transmural + granulomas = textbook Crohn's. Even with bloody stool — Crohn's CAN bleed, especially Crohn's colitis. The skip pattern is the key.
Tricky. Patchy distribution with only mucosal changes. This is indeterminate colitis territory — about 10-15% of IBD cases can't be neatly classified. Boards usually won't test this, but know it exists.
Terminal ileum involvement. Any complications?
Crohn's Disease. Terminal ileum + fistulas/strictures = Crohn's. These complications happen because the inflammation is transmural — it goes deep enough to create tunnels (fistulas) or narrow the lumen (strictures). UC can't do this because it stays superficial.
Crohn's Disease. Terminal ileum damage causes B12 deficiency (B12 is absorbed here) and bile salt malabsorption (leading to fat malabsorption and steatorrhea). UC doesn't cause these because it only affects the colon.
Patchy colonic disease with non-bloody stools. Key finding?
Crohn's colitis. Cobblestoning = alternating deep ulcers and normal mucosa creating a cobblestone pattern. Creeping fat = mesenteric fat wrapping around bowel. Both are transmural signs that UC cannot produce.
Crohn's Disease. Perianal disease (fissures, fistulas, skin tags) is a strong Crohn's clue. Happens in ~30% of Crohn's patients. UC does NOT cause perianal fistulas because it never goes transmural.
Crohn's colitis mimics UC. When Crohn's is limited to the colon, it CAN look continuous. The giveaway is histology: non-caseating granulomas + transmural inflammation = Crohn's regardless of location. Location suggests; histology confirms.
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.