Which of the following is an example of a physical staining method?

• A. Toluidine Blue
• B. Verhoeff-Van Gieson
• C. Methenamine Silver
• D. Oil Red O

Answer: (D)

Stains that rely on physical properties stain components by solubility or solvent affinity rather than through a chemical reaction with the tissue. Oil Red O fits this, because it is lipid-soluble and stains neutral lipids by dissolving into lipid droplets and then taking up the dye, which colors them red. This method works on frozen sections where lipids are preserved, since routine paraffin processing would remove them. Other options use chemical interactions—toluidine blue binds to tissue components through electrostatic and metachromatic effects, Verhoeff-Van Gieson relies on metal-hematoxylin/acid-metal reactions, and methenamine silver deposits silver through chemical reactions—so they are chemical staining methods.