Dimedone's role in PAS staining is to block reaction of non-glycogen PAS-positive substances while preserving glycogen staining. Which compound is used for this purpose?

• A. Dimedone
• B. Iodine
• C. Sodium periodate
• D. Schiff's reagent

Answer: (A)

In PAS staining, periodic acid oxidizes vicinal diols to form aldehydes, and Schiff’s reagent then produces the magenta color at those aldehyde sites. Because many non-glycogen substances (like mucopolysaccharides, glycoproteins, etc.) also have structures that can become aldehydes, background staining is a concern. Dimedone is used as an aldehyde-blocking agent: it reacts selectively with aldehyde groups to form stable adducts, effectively masking these aldehydes so Schiff’s reagent can no longer react with them. This reduces non-glycogen PAS-positive background while still allowing glycogen-related aldehyde formation to be detected under the protocol, giving glycogen staining with greater specificity.

The other options aren’t used for this blocking purpose: iodine is a stain for starch, sodium periodate is the oxidant that creates aldehydes in PAS, and Schiff’s reagent is the staining reagent itself.