Ingredients in Schiff reagent.

• A. Basic fuchsin, HCl, sodium metabisulfite, DI
• B. Hematoxylin, eosin, water
• C. Crystal violet, acetic acid, glycerol
• D. Sodium thiosulfate, silver nitrate, water

Answer: (A)

Schiff reagent is a decolorized dye used to reveal aldehyde groups created in the periodic acid–Schiff reaction. The dye behind it is basic fuchsin, which is reduced (decoulorized) by sulfite in an acidic environment so it remains colorless until it encounters aldehydes. The acid, provided by hydrochloric acid, creates the proper conditions for the dye to exist in a form that can be reactivated by aldehydes. The sulfite source, sodium metabisulfite, is what converts the fuchsin to its colorless form and maintains the Schiff reagent in that decolorized state until the aldehyde-containing substrate is present. Distilled water serves as the solvent to bring these components together at the right concentration. In the PAS workflow, periodic acid oxidizes carbohydrates to aldehydes, and the Schiff reagent then regains its magenta color upon reaction, signaling the presence of those carbohydrates. Other option sets correspond to different stains or reagents used in histology, not Schiff reagent.