What are the three ingredients in the B5 fixative?

• A. Mercuric chloride, Sodium acetate, Formaldehyde
• B. Zinc chloride, Sodium acetate, Formaldehyde
• C. Mercuric chloride, Sodium phosphate, Formaldehyde
• D. Mercuric chloride, Sodium acetate, Ethanol

Answer: (A)

B5 fixative is defined by its trio of ingredients: mercuric chloride, sodium acetate, and formaldehyde. Mercuric chloride provides strong protein fixation and excellent preservation of nuclear detail by crosslinking cellular components. Sodium acetate acts as a buffer, helping to maintain a stable pH so that fixation proceeds consistently and artifacts are minimized. Formaldehyde serves as the aldehyde fixative that further crosslinks proteins, locking cellular structures in place for subsequent staining.

Other combinations don’t fit because they replace the buffering component with something like sodium phosphate or swap in zinc chloride or ethanol. Sodium acetate is the typical buffer in B5, not sodium phosphate, and ethanol does not provide the same crosslinking fixative action as formaldehyde.