A 28-year-old man from Delhi presents with a 6-month history of bloody diarrhea, abdominal pain, and weight loss of 8 kg. Colonoscopy reveals continuous inflammation of the mucosa and submucosa limited to the colon, with a cobblestone appearance and skip lesions. Deep ulcers are noted, and biopsy shows transmural inflammation with non-caseating granulomas. Which of the following findings would be MOST helpful in distinguishing this patient's condition from ulcerative colitis?