Name a stain that requires mounting with an aqueous mounting medium.

• A. Oil Red O
• B. Hematoxylin
• C. Eosin
• D. Methylene blue

Answer: (A)

Oil Red O is a lipid-staining dye that highlights neutral lipids in tissue. Lipids are lost or dissolved when tissue is processed through standard paraffin techniques that use organic solvents, so this stain is applied to frozen sections and then mounted with an aqueous mounting medium to keep the lipids and the red stain visible. Using a traditional xylene-based mount would dissolve the lipids and wash away the color, nullifying the stain. Other common stains like hematoxylin and eosin are used on paraffin sections and typically require non-aqueous mounting after dehydration and clearing, so they don’t rely on an aqueous mount. Methylene blue isn’t specifically tied to maintaining lipid content, so it isn’t defined by this mounting requirement.