A dye that is not water soluble and used to demonstrate fat is:

• A. Oil Red O
• B. Nile Blue Sulfate
• C. Sudan Black B
• D. Sudan III

Answer: (A)

Demonstrating fat relies on a dye that remains in lipids and does not wash away with water. Oil Red O is a fat-soluble dye that dissolves in lipids and stains neutral fats a bright red-orange. Because routine paraffin processing removes lipids, this stain is typically applied to frozen tissue sections (where lipids are preserved) and used with a non-aqueous solvent like isopropanol. The result is clear visualization of fat deposits as red areas against a counterstain such as hematoxylin. While other lipid stains (like Sudan Black B or Sudan III) can also highlight fats, Oil Red O is the classic, most straightforward choice for demonstrating neutral fats in histology. Nile Blue Sulfate, being water-soluble, isn’t used for this purpose in the same way.