During a pathology review session, two biopsy specimens from patients with inflammatory bowel disease are compared. Specimen A shows inflammation limited to the mucosa with crypt distortion, crypt abscess formation, and surface ulceration. Specimen B shows patchy full-thickness inflammation with fissuring ulcers and cobblestone appearance. Which histological finding most reliably distinguishes Specimen B (Crohn disease) from Specimen A (ulcerative colitis)?