Which substance is demonstrated by Congo red staining showing apple-green birefringence under polarized light?

• A. Lipids
• B. Glycogen
• C. Amyloid
• D. Mucin

Answer: (C)

Concerning amyloid, Congo red binds to the fibrils that have a cross beta-pleated sheet structure. When viewed with polarized light, the dye-fibril complex exhibits birefringence that appears apple-green, which is a classic diagnostic sign of amyloid deposits in tissue. Lipids, glycogen, and mucin do not produce this apple-green birefringence with Congo red—lipids are shown with lipid stains, glycogen with PAS, and mucin with Alcian blue or mucicarmine—so the apple-green birefringence under polarized light specifically points to amyloid.