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

• A. Mucin
• B. Collagen
• C. Elastin
• D. Keratin

Answer: (A)

Mucin is identified by its red color when stained with mucicarmine because this stain binds specifically to acidic mucopolysaccharides in mucin, producing a bright red result. In tissues, mucin-rich goblet cells or secretions stand out distinctly with this red stain. The other substances have different color patterns with standard or special stains: elastin is highlighted darkly with stains like Verhoeff-Van Gieson, collagen typically shifts blue or green with Masson's trichrome or pinkish with other protocols, and keratin tends to be eosinophilic pink with H&E. Therefore, the red-stained material points to mucin.