Which additive fixative that contains aldehyde is listed among the options?

• A. Formalin
• B. Bouin's
• C. Osmium Tetroxide
• D. Alcohol

Answer: (A)

Aldehyde-based fixatives preserve tissue structure by crosslinking proteins. Formalin is a classic aldehyde fixative—it's an aqueous solution of formaldehyde (usually buffered) that quickly penetrates tissue and forms methylene bridges between protein molecules, locking in cellular detail for subsequent staining. That crosslinking action is why formalin is the standard aldehyde-containing additive fixative among the options.

Osmium tetroxide fixes primarily by reacting with lipids and acts as an oxidizing fixative, not an aldehyde. Alcohol fixes tissues by precipitating proteins and dehydrating sections, without aldehyde crosslinking. Bouin's solution is a complex mixture that includes formaldehyde but is dominated by picric and acetic acids, so it’s not the straightforward aldehyde fixative represented by formalin.