A 63-year-old woman has nephrotic-range proteinuria, macroglossia, and a small serum M-spike of 0.9 g/dL. Bone marrow shows 12% plasma cells. Renal biopsy stained with Congo red shows amorphous eosinophilic deposits in glomeruli and vessels. Which finding confirms the nature of these deposits?

• A Positive periodic acid-Schiff staining
• B Apple-green birefringence under polarized light ✓
• C Silver stain positivity of the deposits
• D Strong CD138 expression in the deposits

Explanation

Congo red binds the beta-pleated sheet fibrils of amyloid and produces characteristic apple-green birefringence under polarized light, the confirmatory stain for amyloidosis of any type. In this setting the monoclonal light chains from the plasma cell clone form AL amyloid. PAS and silver stains highlight glycogen and basement membranes respectively, and CD138 marks plasma cells themselves, not the deposited fibrillar protein.

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.