Electron microscopy of a renal biopsy from a patient with membranoproliferative glomerulonephritis (MPGN) shows dense deposits within the GBM lamina densa forming ribbon-like, sausage-shaped osmiophilic deposits. C3 is strongly positive on IF but no immunoglobulin staining. The diagnosis and underlying mechanism are:

• A Dense deposit disease (C3 glomerulopathy) — C3 nephritic factor activating alternative complement pathway ✓
• B MPGN type I — immune complex deposition activating classical pathway
• C MPGN type III — hybrid deposits in all GBM layers via lectin pathway
• D Fibrillary glomerulonephritis — organized fibrils on EM with polyclonal IgG deposition

Explanation

Dense deposit disease (DDD), now classified under C3 glomerulopathy per MPGN reclassification, shows pathognomonic highly osmiophilic dense deposits within the GBM lamina densa on EM. Immunofluorescence shows C3 only (no immunoglobulins), reflecting dysregulation of the alternative complement pathway — most commonly by C3 nephritic factor (C3NeF), an autoantibody that stabilizes the C3 convertase (C3bBb), causing continuous uncontrolled C3 activation. MPGN type I shows subendothelial immune complex deposits (IgG+C3 on IF) activating the classical pathway.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.