Congo red staining with apple-green birefringence under polarised light is the gold-standard histochemical identification for amyloid. Which structural property of amyloid fibrils explains this birefringence?

• A Alpha-helical secondary structure with parallel hydrogen bonds
• B Random coil conformation allowing non-specific Congo red binding
• C Collagen triple helix structure binding Congo red in a similar manner to fibrosis
• D Cross-beta-pleated sheet architecture aligned perpendicular to the fibril long axis ✓

Explanation

All amyloid fibrils, regardless of precursor protein, share the cross-beta-pleated sheet structure: beta-strands run perpendicular to the long axis of the fibre, connected by hydrogen bonds parallel to the fibre axis. This highly ordered, regular quaternary structure allows Congo red molecules to align systematically with the fibrils, creating optical anisotropy and the characteristic apple-green birefringence under polarised light. Alpha-helical structures do not bind Congo red in this manner. Random coil and collagen triple helices have different periodicity and do not produce apple-green birefringence.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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