Complement your knowledge with a rare cause of pauci-immune glomerulonephritis.

RPGN can be subdivided into three categories on an immunopathologic basis: pauci-immune glomerulonephritis (PIGN), anti-glomerular basement membrane glomerulonephritis (anti-GBM disease), or immune complex-mediated glomerulonephritis (GN). PIGN is the most common cause of RPGN (80% of cases). The most common etiology of PIGN is anti-neutrophil cytoplasmic antibody (ANCA)-associated GN, which accounts for up to 90% of PIGN. PIGN is unique in that few to no immunoglobulin deposits are seen on glomerular immunofluorescence (IF) and electron microscopy (EM), but it is important to remember that dysregulation of the alternative pathway may result in the deposition of complements leading to inflammatory injury even in PIGN. Membranoproliferative glomerulonephritis (MPGN) is a rare, primary glomerular disorder. Both ANCA-associated GN and complement-mediated MPGN will lack immunoglobulin staining on immunofluorescence (IF) and thus present as PIGN on pathologic examination. This may lead to occurrences where these entities mimic one another, therefore necessitating heightened suspicion and close pathologic examination of a renal biopsy with electron microscopy to differentiate the diagnoses. This case highlights a rare case of C3 GN mimicking ANCA-associated GN.