Which preservative is known to be mercury-based?

Thimerosal is a mercury-based preservative commonly used in some vaccines and medical products to prevent bacterial and fungal contamination. It is an ethylmercury compound, which means it contains mercury in a form that is significantly less toxic than the methylmercury found in some environmental sources. Due to its antimicrobial properties, thimerosal has been employed in various pharmaceutical formulations.

The other options listed do not contain mercury. Propylene glycol is a synthetic compound used as a solvent and humectant in various products. Sorbic acid is a natural preservative effective against mold and yeast. Hydrogen peroxide is a strong oxidizing agent often used as a disinfectant but does not serve a preservative role in the way thimerosal does. Thus, thimerosal is the clear choice when identifying a mercury-based preservative.