A 58-year-old man presents with nephrotic syndrome (proteinuria 5.2 g/day, serum albumin 2.1 g/dL, edema). Kidney biopsy electron microscopy shows the structure marked **A** (subepithelial electron-dense deposits) with characteristic spike-like GBM projections. Immunofluorescence demonstrates granular IgG4 deposition along the GBM. PLA2R antibody testing is positive. Which of the following best describes the pathophysiology underlying the formation of the deposits marked **A**?
A. Antibody-mediated cross-linking of glomerular basement membrane collagen with neutrophil infiltration
C. In situ formation of immune complexes along the GBM with IgG1-IgG3 predominance
D. Circulating immune complex deposition with complement-mediated glomerular injury and proliferation
Explanation
Why IgG4-mediated autoimmune complex deposition targeting podocyte phospholipase A2 receptor antigen is right
The subepithelial electron-dense deposits marked A in primary membranous nephropathy are pathognomonic for IgG4-mediated autoimmune disease. The positive PLA2R antibody and IgG4 granular pattern on immunofluorescence confirm that these deposits represent immune complexes formed between circulating anti-PLA2R IgG4 antibodies and the PLA2R antigen expressed on the podocyte surface. This in situ immune complex formation leads to complement activation (C3 deposition in "pinhead" pattern) and progressive GBM thickening without cellular proliferation, as described in Harrison's 21e and KDIGO 2021 guidelines. The IgG4 subclass predominance distinguishes primary MN from secondary forms.
Why each distractor is wrong
Circulating immune complex deposition with complement-mediated glomerular injury and proliferation: This describes post-infectious glomerulonephritis or lupus nephritis (Class III/IV), which present with cellular proliferation on light microscopy and diffuse endocapillary inflammation. MN is characterized by the absence of proliferation and subepithelial (not subendothelial) deposits.
In situ formation of immune complexes along the GBM with IgG1-IgG3 predominance: While in situ formation is correct, IgG1-IgG3 predominance is characteristic of secondary MN (associated with infections, malignancies, autoimmune diseases like lupus). Primary MN is defined by IgG4 predominance and PLA2R positivity.
Antibody-mediated cross-linking of glomerular basement membrane collagen with neutrophil infiltration: This mechanism describes anti-GBM disease (Goodpasture syndrome), which presents with linear IgG deposition on immunofluorescence, rapid crescentic glomerulonephritis, and pulmonary hemorrhage—not the granular pattern and subepithelial deposits of MN.
High-YieldNEET PG
Primary membranous nephropathy = PLA2R-IgG4 autoimmune disease with subepithelial deposits; secondary MN = IgG1-IgG3 with systemic disease or malignancy.
