The preferred fixative for the technique shown in the image is Bouin solution. Which of the following is the fixative used?

• A. Bouin solution
• B. Formalin
• C. Alcohol
• D. Glutaraldehyde

Answer: (A)

Bouin solution is chosen because the technique shown relies on a fixative that preserves delicate tissue architecture while enhancing contrast for a connective-tissue–focused stain, such as a trichrome method. Bouin’s fixative is a mixture of picric acid, formaldehyde, and acetic acid in water. It penetrates quickly and fixes tissues in a way that makes collagen and other connective-tissue elements stand out distinctly when the sections are stained with trichrome dyes. The acetic acid component sharpens nuclear detail, and the picric acid component helps intensify collagen staining, giving the crisp differentiation these stains require.

After fixation, excess picric acid should be washed out (often with 70% ethanol) before dehydration and paraffin embedding to avoid yellow background staining and handling hazards. Formalin alone doesn’t provide the same enhancement of connective-tissue differentiation for trichrome procedures, alcohol fixes tissues but can shrink and distort morphology and doesn’t give the same staining contrast, and glutaraldehyde is mainly used for electron microscopy rather than routine light microscopy.