Which stain is used to identify acid mucins?

• A. Alcian blue pH 2.5
• B. Mayer's mucicarmine
• C. Periodic acid–Schiff
• D. Crystal violet

Answer: (A)

Acid mucins are mucopolysaccharides that carry negative charges from sulfate and carboxyl groups, so they bind dyes that interact with those charges. Alcian blue at pH 2.5 is used because, under this slightly acidic condition, it selectively binds to sulfated and carboxylated mucins, turning them blue. This makes it the best choice for identifying acidic mucins, distinguishing them from neutral mucins that PAS highlights in magenta. Other stains have different targets: mucicarmine colors mucins pink to red, while crystal violet is a general dye and does not specifically identify mucin types.