Which fixative contains picric acid, formalin, and acetic acid?

• A. Bouin
• B. Carnoy's
• C. Zenker's
• D. Formalin

Answer: (A)

Bouin's fixative is a mixture of picric acid, formalin, and acetic acid. The key clue is the combination of picric acid with formaldehyde (formalin) and acetic acid, which uniquely identifies Bouin's solution. Formalin cross-links proteins to preserve structure, acetic acid helps preserve nuclear detail and soft tissue, and picric acid provides rapid fixation and a distinctive yellow coloration that reflects its presence. Other fixatives use different components—for example, Carnoy's uses ethanol and chloroform with acetic acid, Zenker's uses mercuric chloride and potassium dichromate with acetic acid, and plain formalin is simply formaldehyde in water. So the described mixture corresponds to Bouin's fixative.