Which structure is stained red in the described image?

• A. The basement membrane
• B. Nuclei
• C. Cytoplasm
• D. Mitochondria

Answer: (A)

Red staining in this context points to the basement membrane. It’s the thin extracellular layer that sits between epithelial cells and the underlying connective tissue, rich in carbohydrate-containing molecules like glycoproteins and proteoglycans. Stains such as PAS react with those carbohydrates, giving the basement membrane a bright red/pink color and making this boundary stand out. Nuclei would be blue/purple with common stains, and cytoplasm is typically pink but distributed within cells rather than forming a distinct boundary; mitochondria don’t show up as a specific red structure in these stains.