What is used to remove mercury pigment from slides fixed with mercury-containing fixatives?

• A. Iodine-sodium thiosulfate
• B. Sodium thiosulfate
• C. Hydrogen peroxide
• D. Sodium hypochlorite

Answer: (A)

This question tests the demercuration step used to remove mercury pigment from slides fixed with mercury-containing fixatives. Mercury pigments can obscure tissue details, so a clearing step is needed before staining.

The best approach uses iodine followed by sodium thiosulfate. First, iodine oxidizes and converts mercurial residues into a soluble mercuric-iodide complex. Then sodium thiosulfate dissolves that complex, effectively removing the mercury pigment from the tissue and leaving a clean background for subsequent staining. This two-step sequence is specific for demercuration and minimizes damage to the tissue.

Other agents listed aren’t as effective for this purpose: hydrogen peroxide and sodium hypochlorite are strong oxidizers that can damage tissue and staining rather than specifically clearing mercury pigment, and sodium thiosulfate alone lacks the initial step to form the soluble mercury complex.