What is in B-5 fixative?

• A. Formaldehyde and mercuric chloride
• B. Acetic acid
• C. Chloroform
• D. Picric acid

Answer: (A)

B-5 fixative is a classic mixture that combines formaldehyde with mercuric chloride. Formaldehyde acts as a crosslinking fixative, stabilizing proteins to preserve overall tissue structure for paraffin embedding. Mercuric chloride serves as a mordant, enhancing staining by improving nuclear detail and the affinity of dyes like hematoxylin. Acetic acid, chloroform, and picric acid are associated with other fixatives or roles (such as tissue softening, clearing, or different staining properties) and are not components of B-5. Thus, the defining combination in B-5 is formaldehyde and mercuric chloride.