Which stain is primarily a fluorescence technique?

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

Answer: (A)

Fluorescence-based staining relies on dyes that emit light when excited by a specific wavelength, rather than just producing a color visible with standard light. Thioflavin T is the fluorogenic dye here: it becomes highly fluorescent when it binds to beta-amyloid fibrils, allowing visualization of those deposits under a fluorescence microscope. Typically, it is excited around 440–450 nm and emits near 482–490 nm, so amyloid appears bright under the appropriate filter.

Congo red detects amyloid by binding and showing apple-green birefringence under polarized light, not by fluorescence. Alcian blue and PAS generate color changes seen with ordinary light microscopy, staining mucopolysaccharides and carbohydrates, respectively. So the stain that is primarily a fluorescence technique is Thioflavin T.