Ulcerative colitis always involves the rectum and may extend prox-imally in a continuous distribution.
Crohn disease most commonly involves the distal ileum, but it may involve any portion of the gastrointestinal tract and has "skip lesions."
Because of transmural inflammation, Crohn disease often is complicated by fistula formation.
Toxic megacolon is characterized by dilation of the colon along with systemic toxicity; failure to improve with medical therapy may require surgical intervention.
Both ulcerative colitis and Crohn disease can be associated with extraintestinal manifestations, such as uveitis, erythema nodosum, pyoderma gangrenosum, arthritis, and primary sclerosing cholangitis.
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