Which component is stained red in the image?

• A. Protein
• B. Lipid
• C. Carbohydrate
• D. Nucleic acid

Answer: (B)

Red staining indicates lipid because fat-soluble dyes such as Oil Red O or Sudan stains specifically bind neutral lipids (triglycerides and cholesterol esters) and render them bright red. This visualization is typically seen on frozen sections, since routine paraffin processing removes lipids. Other components have characteristic colors with common stains—proteins often stain pink with eosin, carbohydrates with PAS as magenta, and nucleic acids blue/purple with hematoxylin—so the distinct red signal here points to lipids.