Serum protein electrophoresis in a patient with amyloidosis shows a narrow 'M-spike' in the gamma-globulin region. Congo red staining of a rectal biopsy shows apple-green birefringence under polarised light. What structural property of amyloid fibrils produces this characteristic birefringence?

• A Alpha-helical conformation of amyloid protein binds Congo red at specific hydrophobic pockets
• B Beta-pleated sheet secondary structure of the fibrils aligns Congo red dye molecules in an ordered, anisotropic array ✓
• C Calcium phosphate crystals within fibrils diffract polarised light
• D Collagen triple helix structure co-deposited with amyloid rotates plane of polarised light

Explanation

All amyloid fibrils share a cross-beta pleated sheet quaternary structure where individual peptide chains run perpendicular to the long axis of the fibril. Congo red dye intercalates between the beta-sheet strands in an ordered, anisotropic manner; this molecular ordering produces the characteristic apple-green birefringence under polarised light. This birefringence is diagnostically pathognomonic for amyloid regardless of the precursor protein type (AL, AA, TTR, etc.). The cross-beta sheet architecture also explains amyloid's resistance to proteolytic degradation.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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