Which staining reagent stains lipids and requires mounting with an aqueous medium?

• A. Oil Red O
• B. Hematoxylin
• C. Eosin
• D. Mason's trichrome

Answer: (A)

Staining lipids requires preserving the lipid content in tissue, which routine paraffin processing can remove because it uses organic solvents. Oil Red O is a fat-soluble dye that specifically binds neutral lipids, making them appear red. Because lipids and the lipid-dye complex can be dissolved by alcohols and other solvents, this stain is used on fresh or frozen tissue sections and mounted with an aqueous medium to keep the lipids in place. The other stains target different tissue components—nuclei, cytoplasm, or connective tissue—and do not specifically label lipids, so they don’t require an aqueous mounting in the same way.