Which dye could be used as an alternative to Congo red to visualize amyloid in tissue?

• A. Crystal violet
• B. Toluidine blue
• C. Masson's trichrome
• D. Prussian blue

Answer: (A)

Amyloid has a distinct affinity for certain dyes, allowing it to be seen in tissue sections. Congo red is the classic stain because amyloid crystals bound to it show apple-green birefringence under polarized light. When Congo red isn’t available, crystal violet can be used as an alternative stain; it binds to the amyloid deposits and makes them stand out with a purple color under standard light microscopy. The other dyes shown don’t specifically highlight amyloid—toluidine blue is a general stain with metachromasia but not an established amyloid stain, Masson's trichrome emphasizes collagen and other fibers, and Prussian blue detects iron. Thus crystal violet is the best alternative among these options for visualizing amyloid in tissue.