Dimedone's role in PAS staining is to block reaction of non-glycogen PAS-positive substances while preserving glycogen staining. Which statement best describes its function?

• A. Block the reaction of non-glycogen PAS-positive substances while preserving glycogen staining
• B. Increase PAS staining intensity
• C. Stain glycogen directly
• D. Act as a counterstain when glycogen is absent

Answer: (A)

Dimedone serves as a selective blocker of aldehyde reactivity in PAS staining. After periodic acid oxidation, Schiff reagent binds to aldehyde groups on many PAS-positive substances, producing color. Dimedone binds to those aldehyde groups on non-glycogen substances, forming stable adducts that prevent Schiff from reacting with them. This masking reduces background staining while leaving the glycogen-derived aldehydes available to react with Schiff, so glycogen staining is preserved. It’s not about increasing intensity, staining glycogen directly, or acting as a counterstain.