Under polarized light, Congo red–stained amyloid shows which characteristic?

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

Answer: (A)

The key idea is the diagnostic signal produced when Congo red–stained amyloid is viewed under polarized light. The amyloid fibrils have a highly ordered beta-pleated sheet structure, and Congo red binds along these fibrils in a way that, with crossed polarizers, splits light to produce an apple-green color. This apple-green birefringence is the classic hallmark used to confirm amyloid in tissue sections.

Blue birefringence isn’t expected with Congo red–stained amyloid; no birefringence would suggest the sample lacks the organized fibrillar arrangement, and red fluorescence isn’t a typical result under polarized light for Congo red–amyloid staining.