A 36-year-old woman has abdominal pain and ulcers. What is the most probable diagnosis?

The most probable diagnosis is Crohn's disease, particularly considering the symptoms of abdominal pain and ulcers. This inflammatory bowel disease can affect any part of the gastrointestinal tract, from the mouth to the anus, and is characterized by transmural inflammation. The presentation of abdominal pain, combined with the formation of ulcers, fits well within the clinical picture for Crohn's disease, which is known to cause significant gastrointestinal complications and can lead to strictures, fistulas, and, in some cases, bowel obstruction.

In contrast, ulcerative colitis primarily affects the colon and presents with continuous inflammation of the colonic mucosa, leading to bloody diarrhea rather than isolated abdominal pain with ulceration at various sites. Diverticulitis is usually associated with localized pain and possibly fever due to diverticular inflammation or infection, and it typically presents with tenderness in the left lower quadrant in adults. Irritable bowel syndrome (IBS) is functional and doesn’t cause physical changes like ulcers and has different characteristics, such as alternating diarrhea and constipation, without the inflammatory component present in Crohn’s disease.

Given the specific symptoms highlighted, Crohn's disease stands out as the most probable diagnosis.