Fig 1: Representative findings of the kidney biopsy. (A-D) The microscopic findings showed nodular glomerulosclerosis with lobulation. The cells infiltrating the glomerular capillary contained fuchsin-positive and periodic acid–Schiff–negative granules. (A) Hematoxylin-eosin stain: original magnification, ×400. (B) Periodic acid–Schiff stain: original magnification, ×400. (C) Masson trichrome stain: original magnification, ×400. (D) Periodic acid–methenamine–silver stain: original magnification, ×400. (E) The mesangial nodules yielded negative results for direct fast scarlet staining, and some eosinophils appeared to be scattered in the glomeruli (Direct fast scarlet stain, Muto Pure Chemicals Co, Ltd: original magnification, ×400). (F) CD34-positive endothelial cells are located in the center of nodular lesions (staining for CD34, Nichirei). (G) The cells infiltrating the glomerular capillaries were CD68 positive (staining for CD68, Dako M0876 PG-M1 clone: original magnification, ×400). (H, I) Immunohistochemical staining of the cytoplasm of capillary histiocytes tested positive for kappa light chains, whereas they tested negative for lambda light chains. (H) Staining for kappa light chains, Nichirei 713891 L1C1 clone: original magnification, ×400. (I) Staining for lambda light chains, Nichirei 713901 HP6054 clone: original magnification, ×400. (J-L) Additional paraffin immunofluorescence staining showed positivity for immunoglobulin (Ig)G and kappa in the histiocytes of glomerular capillaries but negativity for lambda. The glomerular basement membrane and tubular basement membrane were negative for kappa and lambda (immunofluorescence on pronase-digested paraffin sections). (J) IgG, Dako F020202: original magnification, ×400. (K) Kappa, SouthernBiotech 2060-02: original magnification, ×400. (L) Lambda, SouthernBiotech 2070-03: original magnification, ×400. (M-O) Electron microscopy. A large mesangial nodular lesion without any powdery electron-dense deposition. (M) The glomerular subendothelial foam cells (histiocytes) included electron-dense granules and vacuoles. (N) Layered electron-dense deposits were seen in the glomerular basement membrane, inconsistent with monoclonal immunoglobulin deposition disease. Foam cell infiltration was observed below the tubular basement membrane. (O) Few vacuoles were observed in the tubular epithelial cells. (P-R) Immunoelectron microscopy. Immunogold labeling for kappa light chains yielded positive results in lysosomes, whereas they yielded negative results for lambda (P: kappa, Q: lambda, R: kappa; primary antibody for kappa/lambda light chains, Dako; 10-nm gold-conjugated secondary antibody; BBI solutions). (R) Positive kappa signals were not observed along the glomerular basement membrane, a finding inconsistent with monoclonal immunoglobulin deposition disease.