IgG4-related disease masquerading as malignancy in the abdomen: A diagnostic challenge.

IgG4-related disease (IgG4-RD) is a fibro-inflammatory autoimmune disorder with an incompletely understood pathophysiology. This condition can affect a wide range of organs, and its hallmark features include tissue damage caused by an inflammatory, mass-forming process, often accompanied by significant fibrosis, venulitis, and a dense lymphoplasmacytic infiltrate rich in IgG4-positive plasma cells. In this report, we describe a rare and atypical case of IgG4-RD in a male in his late twenties who presented with abdominal pain and a palpable bulge. Imaging studies, including abdominal CT, revealed a terminal ileal mass with associated lymphadenopathy, initially raising strong clinical suspicion for a small bowel malignancy. However, further evaluation through colonoscopy and biopsy ultimately confirmed the diagnosis of IgG4-RD. This case underscores the diagnostic complexity of IgG4-RD, especially when it mimics malignancy, and highlights the importance of thorough investigation to ensure accurate diagnosis and appropriate management.