Which feature is observed when amyloid stained with Congo red is viewed under polarized light?

• A. Apple-green birefringence
• B. Red fluorescence
• C. Brown staining
• D. No birefringence

Answer: (A)

When amyloid is stained with Congo red and viewed under polarized light, it shows apple-green birefringence. This happens because amyloid has a highly organized beta-pleated sheet structure that binds Congo red in a way that creates an anisotropic, crystal-like arrangement. Under polarized light, this organized complex splits light into two rays that travel at different speeds, producing the characteristic green color. This optical signature is a classic diagnostic feature of amyloid in tissue. Other possibilities, like red fluorescence or brown staining, do not reflect this specific birefringent property, and no birefringence would not fit the observed plate-like, color-shifting effect seen with Congo red–amyloid deposits.