Which fixative is not typically used with best carmine?

• A. Osmium tetroxide
• B. Formalin
• C. Carnoy
• D. Bouin

Answer: (A)

Best carmine is used to highlight carbohydrate-rich components in tissue, such as glycogen and mucopolysaccharides, so the fixing agent must preserve those substances without masking them. Osmium tetroxide is a lipid-focused fixative mainly used for electron microscopy; it fixes and stains lipids in a way that isn’t compatible with the carbohydrate targets of Best carmine. That’s why it’s not typically used with this stain. In contrast, formalin, Carnoy, and Bouin are standard fixatives that preserve tissue morphology and carbohydrate-containing elements well enough for Best carmine to produce clear, specific staining.