Which stain is primarily a fluorescence technique?

• A. Thioflavin T
• B. Congo red
• C. PAS
• D. Alcian blue

Answer: (A)

Fluorescence-based staining uses a dye that emits light when it is excited by a specific wavelength, allowing detection with a fluorescence microscope. Thioflavin T binds to amyloid fibrils and, upon excitation, emits bright fluorescence, making it ideal for visualizing amyloid deposits. The other stains here are colorimetric: Congo red is read by polarized light for birefringence, not fluorescence; PAS produces a magenta color through a chemical reaction; Alcian blue colors acidic mucopolysaccharides blue. So thioflavin T is the primary fluorescence technique.