A 68-year-old woman with kappa restricted plasma cell myeloma develops progressive ankle edema and nephrotic range proteinuria. Urine dipstick is strongly positive for protein, but serum albumin electrophoresis shows no intact immunoglobulin abnormality in urine. Renal biopsy stained with Congo red shows amorphous eosinophilic deposits in glomeruli and vessels. Under polarized light, what appearance of these deposits confirms the diagnosis?

• A Black staining with silver methenamine
• B Bright orange fluorescence with acridine orange
• C Metachromatic purple staining with toluidine blue
• D Apple-green birefringence ✓

Explanation

AL amyloidosis arises when monoclonal light chains produced by the plasma cell clone misfold into beta-pleated sheet fibrils that deposit in kidneys, heart, nerves and liver. Congo red stains these deposits salmon pink on routine light microscopy and gives characteristic apple-green birefringence under polarized light, which is the diagnostic gold standard. Silver methenamine highlights basement membranes, and metachromatic staining is a feature of mast cell granules and cartilage matrix.

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.