Osmium tetroxide is used as a fixative for which preparation?

• A. Paraffin processed fat
• B. Aldehyde fuchsin
• C. Gomori Trichrome
• D. Phosphotungstic acid-hematoxylin

Answer: (A)

Osmium tetroxide fixes lipids. It reacts with the double bonds in fatty acids, creating stable, lipid-rich complexes that resist extraction during processing. This makes it the fixative of choice when the goal is to preserve fat in tissue that will be paraffin-embedded, so fat can be demonstrated or studied after processing. The other items are staining reagents, not fixatives: aldehyde fuchsin stains elastic fibers, Gomori trichrome differentiates muscle and connective tissue components, and phosphotungstic acid-hematoxylin provides contrast staining—not lipid fixation.