Medical mirroring: granulomatosis with polyangiitis (formerly Wegener's) mimicking immunoglobulin-G4 related disease.

Granulomatosis with polyangiitis (GPA; formerly Wegener's) can present with clinical and histopathological features similar to those of immunoglobulin-G4 related disease (IgG4-RD), a recently described fibro-inflammatory condition. The ability of these two distinct entities to mimic each other closely creates significant pitfalls in diagnosis. We present a unique case in which GPA presented as a peri-aortic fibrotic mass in the retroperitoneum.