A 22-year-old man comes to his physician because of worsening crampy right lower quadrant pain and loose, non-bloody stools four to five times daily for the past 3 months. He has lost 9 pounds without dieting. Temperature is 37.6°C; mild right lower quadrant tenderness is present without rebound. Colonoscopy reveals a normal-appearing rectum. The terminal ileum and cecum show raised, edematous mucosal islands separated by deep fissuring ulcers, while adjacent bowel segments appear grossly normal. No pseudomembranes are seen. Biopsy of the affected segments is obtained.
Which of the following biopsy findings would most specifically establish this patient's diagnosis?