In Congo Red-stained amyloid, what optical property is observed under polarized light?

• A. Green birefringence
• B. Pink color
• C. Blue fluorescence
• D. Yellow birefringence

Answer: (A)

Apple-green birefringence under polarized light is the distinctive feature when Congo red–stained amyloid is examined. The dye binds along the aligned beta-pleated sheet structure of amyloid, creating an organized, anisotropic complex. When viewed with polarized light, this arrangement causes double refraction and a green interference color—often described as apple-green—which is a classic diagnostic sign of amyloid deposition. Pink coloration or blue fluorescence are not characteristic of Congo red–stained amyloid under polarized light, and yellow birefringence does not describe this finding.