Which substance is classified as a non-additive fixative?

• A. Ethanol
• B. Formaldehyde
• C. Glutaraldehyde
• D. Osmium tetroxide

Answer: (A)

Non-additive fixatives preserve tissue mainly by coagulation and dehydration rather than by chemically binding new elements to the tissue. Ethanol fixes by removing water and denaturing proteins, causing coagulation without forming covalent bonds or introducing foreign atoms into the tissue structure. In contrast, formaldehyde and glutaraldehyde create covalent cross-links between proteins, effectively adding chemical bonds to tissue components, while osmium tetroxide adds osmium to lipids and acts as a heavy-metal fixative. So ethanol fits the non-additive category because it fixes tissue without adding new chemical bonds or elements.