An 80-year-old man with longstanding multiple myeloma develops macroglossia, carpal tunnel syndrome, and nephrotic syndrome. Renal biopsy shows eosinophilic amorphous deposits in the mesangium and along capillary walls that exhibit apple-green birefringence under polarized light after Congo red staining. Which protein is MOST likely forming these deposits?

• A Immunoglobulin light chains (AL amyloid) ✓
• B Serum amyloid A (SAA) — an acute phase reactant
• C Transthyretin (TTR) — a transport protein
• D Beta-2 microglobulin — from long-term dialysis

Explanation

In the context of multiple myeloma, amyloidosis is caused by misfolded immunoglobulin light chains (most commonly lambda), designated AL amyloid. AL amyloidosis preferentially involves the heart, kidneys, tongue, and peripheral nerves, explaining the macroglossia, nephrotic syndrome, and carpal tunnel. AA amyloid (SAA-derived) complicates chronic inflammatory diseases; TTR amyloid causes familial or senile cardiac amyloidosis; beta-2 microglobulin amyloid is specific to dialysis-related amyloidosis affecting joints.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.