Idiopathic membranoproliferative GN biopsy features include which of the following?

• A. Hypercomplementemia with normal light microscopy
• B. Hypocomplementemia with thickened capillary loops on light microscopy; immune deposits on electron microscopy
• C. Crescent formation with pauci-immune deposits
• D. Subepithelial humps on EM

Answer: (B)

Idiopathic membranoproliferative GN is classically immune complex–mediated with consumption of complement. The biopsy shows thickened capillary loops with a double-contour (tram-track) pattern on light microscopy from mesangial proliferation and GBM splitting, and electron microscopy reveals immune deposits in subendothelial or intramembranous locations. Serum complement levels are typically low due to consumption by the classical pathway activated by these immune complexes. This combination makes the described pattern—hypocomplementemia with thickened capillary loops on light microscopy and immune deposits on electron microscopy—the best fit. The other options point to different processes: hypercomplementemia doesn’t fit MPGN; crescents with pauci-immune deposits suggest RPGN from vasculitis; subepithelial humps are more characteristic of postinfectious GN.