Which chemical listed is a heavy metal salt used as an additive fixative?

• A. Mercuric chloride
• B. Picric acid
• C. Formaldehyde
• D. Osmium tetroxide

Answer: (A)

Additive fixatives rely on introducing metal ions into the tissue to enhance staining and preservation. Heavy metal salts act as mordants, binding to tissue components and creating sites that intensify dye-tissue interactions. Mercuric chloride fits this role because it provides Hg2+ ions that interact with proteins and nucleic acids, helping to stabilize structures and improve contrast for certain stains, especially those that rely on metal-dye interactions. This makes it a classic heavy metal salt used as an additive fixative.

The other substances don’t fit as heavy metal salts: picric acid is an organic precipitating fixative, formaldehyde is a simple aldehyde fixative, and osmium tetroxide is a heavy metal-containing compound but not a salt (it's an oxide) and is used differently, mainly for lipid preservation in electron microscopy.