Small Intestinal Extranodal Marginal Zone Lymphoma Mimicking Crohn's Disease.

Inflammatory bowel disease (IBD) mimics pose a significant diagnostic challenge, particularly in patients with atypical features or refractory disease. Intestinal lymphomas, especially extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT) lymphoma, can closely resemble Crohn's disease clinically, endoscopically, and histologically. We report a 23-year-old woman with a 2-year history of chronic, large-volume diarrhea, weight loss, and malabsorption, initially diagnosed and treated as Crohn's disease. Despite corticosteroids, azathioprine, and biologic therapy, she demonstrated only a partial response. Further evaluation revealed hypoalbuminemia, elevated inflammatory markers, and monoclonal gammopathy on serum protein electrophoresis. Endoscopic and radiological findings showed ileal and jejunal involvement. Histopathological examination demonstrated a dense lymphoplasmacytic infiltrate, and immunohistochemistry revealed CD20 and CD138 positivity with light chain restriction. A final diagnosis of small intestinal extranodal marginal zone lymphoma was established. The patient was treated with combined chemotherapy, with plans to escalate to rituximab-based therapy due to a suboptimal response. This case highlights the importance of reconsidering the diagnosis in patients with presumed IBD who exhibit atypical features, such as severe malabsorption, proximal small bowel involvement, monoclonal gammopathy, or refractory disease. Early recognition of IBD mimics, particularly intestinal lymphomas, is critical to ensure appropriate management and improve outcomes.