The substance stained blue in the image is which of the following?

• A. An acid mucopolysaccharide
• B. Mucin
• C. Collagen
• D. Keratin

Answer: (A)

Blue staining with this histochemical approach indicates the presence of acidic mucopolysaccharides, which are glycosaminoglycans bearing sulfate and carboxyl groups. The stain binds to these negative charges, highlighting proteoglycans and related mucopolysaccharide components as blue. While mucin contains acidic sugar chains, the blue color here reflects the acidic carbohydrate portion targeted by the stain rather than the protein nature of mucin. Keratin and collagen do not present these charged carbohydrate groups under standard staining conditions, so they do not produce the characteristic blue with this stain. Therefore, the blue-stained substance is an acid mucopolysaccharide.