The staining mechanism described as having a positive reaction dependent on the presence of 1,2 glycol grouping and oxidation to aldehydes by periodic acid refers to which stain?

• A. PAS
• B. Best carmine
• C. Iodine method
• D. Mallory stain

Answer: (A)

Periodic acid–Schiff staining works by a chemical sequence that targets carbohydrates and related mucosubstances. Periodic acid oxidizes vicinal 1,2-glycol groups in sugars to form aldehyde groups. Schiff’s reagent then reacts with those aldehydes to produce a vivid magenta color. The description specifically calling out the presence of 1,2 glycol groups and oxidation to aldehydes by periodic acid matches this mechanism exactly, which is why this stain is used for detecting glycogen, mucins, basement membranes, and other carbohydrate-rich substances (with the option to confirm glycogen by including a diastase digestion step). Other stains rely on different chemical principles and do not depend on aldehyde formation from 1,2-diols, so they wouldn’t fit the described mechanism.