A 22-year-old man with inflammatory bowel disease is noted to have a "string sign" in the ileal area on barium enema. In which of the following conditions is this sign most often seen? (See Fig. below.)

The string sign represents long areas of circumferential inflammation and fibrosis. It is seen in the stenotic and non-stenotic phase of Crohn's disease. In addition to the string sign, abnormal puddling of barium and fistulous tracts are other helpful x-ray signs of ileitis. Other radiologic findings in Crohn's disease include skip lesions, rectal sparing, small ulcerations, and fistulas.