Which dye is used to differentiate acidic and neutral lipids?

• A. Nile blue sulfate
• B. Alizarin red
• C. Toluidine blue
• D. Methylene blue

Answer: (A)

When you want to distinguish different lipid classes in tissue, you use a dye that reveals two colors for two lipid types. Nile blue sulfate does exactly that: acidic lipids stain red or pink, while neutral lipids stain blue. This happens because the dye interacts differently with the chemical nature of the lipid classes—the acidic lipids (like phospholipids and related compounds) attract the dye in a way that shifts its color to red, whereas neutral lipids (such as triglycerides and cholesterol esters) yield a blue color with the same stain. This differential coloring makes it possible to visualize and compare the distribution of acidic versus neutral lipids in the same section. The other dyes listed aren’t used for this purpose: alizarin red targets calcium deposits, toluidine blue is a general stain for acidic tissue components and nucleic acids, and methylene blue is a broad basic stain that doesn’t differentiate lipid types.