The most likely diagnosis is Crohn colitis. This patient has severe, patchy colitis with some large, deep ulcers and rectal sparing, which is consistent with Crohn colitis. In Crohn disease, endoscopic findings vary from superficial aphthous ulcers to discrete, deep ulcers that can be linear, stellate, or serpiginous and that may coalesce into a “cobblestone” appearance. Rectal sparing is typical, as are areas of inflammation separated by normal mucosa (known as skip lesions). The ileum is inspected during colonoscopy to detect ileal inflammation characteristic of Crohn disease. Histology may show patchy submucosal inflammation, but more superficial inflammation does not rule out Crohn disease.

Collagenous colitis is a form of microscopic colitis. Microscopic colitis accounts for 10% to 15% of patients with chronic, watery diarrhea. In contrast to inflammatory bowel disease, microscopic colitis is more common in older persons and does not cause endoscopically visible inflammation, as was seen in this patient.

Ischemic colitis is the most common form of intestinal ischemic injury. Approximately 90% of cases occur in patients older than 60 years. Symptoms include sudden abdominal pain and diarrhea followed by rectal bleeding. Ischemic colitis can result in segmental colitis, but it is typically not patchy and would be unusual in a 28-year-old patient.

In ulcerative colitis, inflammation typically begins in the rectum and extends proximally in a circumferential manner. Mild ulcerative colitis is characterized by mucosal edema, erythema, and loss of the normal vascular pattern. More significant disease produces granularity, friability, ulceration, and bleeding. Ulcerative colitis would typically feature rectal involvement and continuous colitis rather than patchy colitis, as was seen in this patient.