The principle of Oil Red O staining is based on:

• A. Dye solubility
• B. Colorimetric change
• C. Enzymatic activity
• D. Nuclear staining

Answer: (A)

Oil Red O works because the dye is fat-soluble and preferentially dissolves into neutral lipids. In tissues with lipid droplets, the lipophilic dye partitions into those lipid-rich regions, staining them red, while aqueous parts of the cell remain unstained. This reliance on solubility in lipids is why Oil Red O staining is performed on frozen sections (to preserve lipids, which are lost during routine paraffin processing) and is not based on a color change driven by a chemical reaction, enzymatic activity, or nuclear staining.