Congo red staining is used to detect amyloid. Which statement is accurate?

• A. It requires polarized light to observe birefringence
• B. It does not require polarized light
• C. It stains lipids
• D. It stains nuclei

Answer: (A)

Congo red binds to the cross beta-pleated sheet structure of amyloid, and when this dye–amyloid complex is viewed under polarized light, it exhibits apple-green birefringence. That birefringence is the diagnostic hallmark because the ordered, amyloid-associated dye alignment changes how light passes through the specimen in a polarized microscope. Polarized light is required to observe this phenomenon; under ordinary light, you won’t see the characteristic green birefringence even though the stain is present. Congo red specifically highlights amyloid, not lipids or nuclei—lipids are typically stained by other methods, and nuclei are stained by hematoxylin or other nuclear stains. Therefore, the accurate statement is that polarized light is needed to observe birefringence.