Which staining reaction forms aldehyde groups through oxidation of carbohydrates for visualization?

• A. PAS
• B. Alcian blue pH 2.5
• C. Colloidal iron
• D. Crystal violet

Answer: (A)

Periodic acid–Schiff reaction relies on oxidizing carbohydrate moieties with periodic acid to create reactive aldehyde groups; Schiff’s reagent then binds these aldehydes to produce a magenta color, allowing visualization of glycogen, mucosubstances, and basement membranes. Alcian blue at pH 2.5 stains acidic mucopolysaccharides by charge interactions rather than aldehyde formation. Colloidal iron detects iron deposits via the Prussian blue reaction, not aldehyde generation. Crystal violet is a basic dye used in Gram staining and general cytology, not involving aldehyde formation from carbohydrates. Thus, the reaction that forms aldehyde groups through oxidation of carbohydrates for visualization is the Periodic acid–Schiff reaction.