A 52-year-old man presents with nephrotic syndrome: proteinuria 12 g/day, serum albumin 2.1 g/dL, and hyperlipidemia. Renal biopsy with silver stain shows diffusely thickened glomerular basement membrane with the characteristic finding marked **A** — silver-positive spikes projecting between subepithelial dense deposits. Serology is positive for anti-PLA2R antibodies. Which of the following best explains the pathogenesis of this morphologic finding?

Explanation

## Why option 1 is right The spike-and-dome appearance on silver stain is pathognomonic for membranous nephropathy (MN) and results from the specific pathogenic mechanism of in situ immune complex formation. Anti-PLA2R antibodies (present in ~70% of primary MN) bind to podocyte surface antigens on the outer (urinary) side of the GBM. This triggers complement activation (C5b-9 membrane attack complex) and podocyte injury. Critically, the subepithelial immune deposits stimulate the GBM to synthesize and deposit new basement membrane material that grows between the deposits (creating "spikes") and eventually surrounds them (creating "domes"), progressively thickening the GBM. This is the defining morphologic hallmark of MN and distinguishes it from all other primary glomerulonephropathies (Robbins 10e Ch 20). ## Why each distractor is wrong - **Option 2 (Mesangial proliferation with double-contour GBM)**: This describes membranoproliferative glomerulonephritis (MPGN) or lupus nephritis class III/IV, not MN. Mesangial proliferation is NOT a feature of MN; the hallmark of MN is preserved mesangial cellularity with isolated GBM thickening. Double-contour ("tram-track") GBM is seen in MPGN, not MN. - **Option 3 (Segmental glomerulosclerosis)**: This describes focal segmental glomerulosclerosis (FSGS), which affects <50% of glomeruli and presents with segmental sclerosis of the glomerular tuft. FSGS does not produce spikes on silver stain and has a different pathogenesis (podocyte cytoskeletal dysfunction, not immune complex deposition). - **Option 4 (Endocapillary proliferation with crescent formation)**: This describes rapidly progressive glomerulonephritis (RPGN) with crescentic morphology, typically seen in ANCA-associated vasculitis or anti-GBM disease. Crescents fill Bowman space and are associated with acute renal failure, not the chronic nephrotic presentation of MN. **High-Yield:** Spike-and-dome on silver stain = membranous nephropathy from subepithelial immune complex deposition with GBM proliferation between deposits; anti-PLA2R serology confirms primary MN and predicts disease activity. [cite: Robbins 10e Ch 20; Harrison 21e Ch 314]