Amyloid detection using Congo red staining yields apple-green birefringence under polarized light.

• A. True
• B. False
• C. It yields yellow birefringence
• D. It yields no birefringence

Answer: (A)

Apple-green birefringence under polarized light is the hallmark of amyloid detection with Congo red. Congo red binds specifically to the beta-pleated sheet structure of amyloid fibrils, and when the stained tissue is viewed under polarized light, this dye-fibril complex produces birefringence that appears an apple-green color. This distinctive optical property is what pathologists rely on to identify amyloid deposits in tissue sections. If the birefringence is not observed, it may indicate absence of amyloid or a technical issue with staining or polarization.