Which statement is true about Congo red staining for amyloid?

• A. It yields apple-green birefringence under polarized light
• B. It stains nuclei blue
• C. It is unaffected by section thickness
• D. It requires silver intensification

Answer: (A)

Congo red binds to the organized beta-pleated sheet structure of amyloid, and when the stained tissue is viewed with polarized light, the dye-amyloid complex exhibits apple-green birefringence. That distinctive green birefringence under polarized light is the diagnostic hallmark that confirms the presence of amyloid deposits, making this the best statement. Under normal light the stained amyloid may look pink to red, but that color alone isn’t specific. Nuclei blue is the domain of hematoxylin staining, not Congo red. Section thickness can influence the visibility of birefringence—too thick or uneven sections can obscure it—so claiming it’s unaffected by thickness isn’t accurate. Silver intensification is a separate technique used with other stains and is not required for Congo red staining of amyloid.