An 61-year-old man with nephrotic-range proteinuria and a serum monoclonal IgG-kappa spike undergoes abdominal fat pad biopsy. Which finding establishes the diagnosis?

• A Linear IgG deposition along the glomerular basement membrane
• B Apple-green birefringence of Congo red stained deposits under polarized light ✓
• C Tram-track splitting of the glomerular basement membrane on silver stain
• D Effacement of foot processes on electron microscopy alone

Explanation

Light chain (AL) amyloidosis is confirmed by Congo red staining showing salmon-pink deposits that give apple-green birefringence under polarized light, reflecting the crossed beta-pleated sheet configuration shared by all amyloid. Subcutaneous fat pad biopsy is a safe site for sampling. Foot process effacement occurs in minimal change disease, tram-tracking in membranoproliferative GN, and linear IgG in anti-GBM disease, none of which involve amyloid deposition.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.