Which fixative containing mercury is commonly used to preserve parasites in stool specimens?

• A. Formalin
• B. Sodium acetate
• C. Buffered glycerol
• D. Polyvinyl alcohol

Answer: (D)

Preserving parasites in stool so they stay morphologically intact for identification relies on fixatives that stabilize the organisms without distorting their structure. Mercury-containing fixatives have long been valued for their strong protein cross-linking, which helps keep fine details of eggs, cysts, and trophozoites visible under the microscope. A polyvinyl alcohol fixative is commonly used for this purpose because, in many formulations, it includes a mercury-containing salt as part of the fixative system. This combination provides good preservation of parasite forms and works well with subsequent staining and preparation for permanent slides. The other options are not mercury-containing fixatives in standard use: formalin is a basic fixative for wet mounts and routine preservation, SAF (sodium acetate-formalin) is mercury-free, and buffered glycerol is used for mounting or lighter preservation rather than mercury-based fixation.