A Drug-Induced Polypoid Lesion

A 79-year-old male patient, with a history of myasthenia gravis, was referred to our department following an episode of fatigue and iron deficiency anemia (hemoglobin concentration 6.7 g/dl, reference range 12.0-16,5 g/dl). Medical therapy included azathioprine and methylprednisolone. An endoscopic work-up with esophagogastroduodenoscopy did not show a clear origin of bleeding. A colonoscopy was therefore performed, which revealed an ulcerating, sessile polypoid lesion protruding from the roof of the ileocecal valve (Figure 1). Biopsies showed heavily inflamed colonic tissue without signs of neoplasia (Figure 2, left), after which immunohistochemical staining was performed (Figure 2, right). What is the underlying cause?