A 56-year-old man presents with progressive ankle oedema. Urine protein is 4.5 g/day with no haematuria. Serum albumin 2.6 g/dL, creatinine 1.3 mg/dL. Skeletal survey shows no lytic lesions, serum calcium is normal, and bone marrow shows 10% plasma cells. Serum free light chain ratio is elevated (lambda-dominant). Renal biopsy shows amorphous eosinophilic deposits in glomeruli and vessel walls. Which stain establishes the nature of these deposits?
- A PAS showing magenta linear thickening of GBM
- B Prussian blue showing perls-positive granules
- C Congo red showing apple-green birefringence under polarized light ✓
- D Silver methenamine showing spike formation on GBM
Correct answer: C. Congo red showing apple-green birefringence under polarized light
Explanation
Monoclonal light-chain (primary AL) amyloidosis presents with nephrotic-range proteinuria out of proportion to renal failure, modest plasmacytosis, and no CRAB features, distinguishing it from symptomatic multiple myeloma. Amyloid is confirmed by Congo red staining with apple-green birefringence under polarized light. Prussian blue detects iron, PAS highlights diabetic nodular glomerulosclerosis, and silver spike formation suggests membranous nephropathy.
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.