What type of microscopy is used to view Congo red–stained specimens when assessing amyloid?

• A. Polarizing
• B. Brightfield
• C. Electron
• D. Confocal

Answer: (A)

Congo red–stained amyloid is assessed with polarized light to reveal apple-green birefringence. The amyloid fibrils have a specific cross beta-pleated sheet structure, and when Congo red binds to these aligned fibrils, it creates anisotropic optical properties. Under polarized light, this junction produces the characteristic green birefringence that is diagnostic for amyloid deposition. Brightfield would show the stain but not the birefringence, electron microscopy would show fibril morphology without relying on Congo red, and confocal microscopy isn’t used for this Congo red–specific diagnostic feature.