A 26-year-old man has had intermittent cramping abdominal pain and low-volume diarrhea for 3 weeks. On physical examination, he is afebrile; there is mild lower abdominal tenderness but no masses and bowel sounds are present. A stool sample is positive for occult blood. The symptoms subside within 1 week. Six months later, abdominal pain recurs with perianal pain. On physical examination, there is now a perirectal fistula. Colonoscopy shows many areas of mucosal edema and ulceration and some areas that appear normal. Microscopic examination of a biopsy specimen from an ulcerated area shows a patchy acute and chronic inflammatory infiltrate crypt abscesses, and noncaseating granulomas. Which of the following underlying disease processes best explains these findings?