A 65-year-old man with nephrotic syndrome and monoclonal gammopathy undergoes renal biopsy. Light microscopy shows amorphous eosinophilic material expanding the mesangium and capillary walls. Which special stain finding confirms the diagnosis?

• A PAS stain demonstrating thickened basement membranes with tram-tracking
• B Silver stain showing spike formation along the GBM
• C Masson trichrome showing fuchsinophilic subendothelial deposits
• D Apple-green birefringence of Congo red stained deposits under polarized light ✓

Explanation

Amyloid is identified by Congo red staining with characteristic apple-green birefringence under polarized light, caused by the crossed beta-pleated sheet configuration of amyloid fibrils. In an older adult with monoclonal gammopathy, the deposits are AL amyloid composed of immunoglobulin light chains. Spikes indicate membranous nephropathy and tram-tracking indicates MPGN, neither of which involves Congo red positive material.

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.