Which fluorescent dye is commonly used to demonstrate amyloid?

• A. Thioflavin T
• B. DAPI
• C. Acridine orange
• D. Ethidium bromide

Answer: (A)

Amyloid detection with fluorescence relies on a dye that binds specifically to the cross-beta sheet structure of amyloid fibrils and whose fluorescence increases when bound. Thioflavin T fits this role well: it interacts with the beta-pleated sheet regions of amyloid fibrils so that its molecular rotation is restricted, resulting in a bright fluorescence signal under standard fluorescence microscopy. This makes Thioflavin T the commonly used fluorescent stain for identifying amyloid deposits in tissue sections or prepared samples, often seen as a vivid blue-green glow after excitation around the blue range.

The other dyes listed are nucleic acid stains and do not selectively highlight amyloid. DAPI binds DNA and emits blue fluorescence, acridine orange stains nucleic acids with color shifts depending on DNA or RNA, and ethidium bromide intercalates DNA and fluoresces under UV light. While useful for cellular or genetic staining, they don’t specifically demonstrate amyloid structures like Thioflavin T does.