MPA is characterized by the presence of anti-MPO (p-ANCA) antibodies, renal and pulmonary involvement, and the absence of granulomatous inflammation on histopathology, a defining feature when compared to GPA and EGPA which, while demonstrating both renal and pulmonary involvement, are characterized by the presence of anti-PR3 (c-ANCA) and granulomatous disease. Here, MPO is linked to granulomatosis with polyangiitis.