Which fixative is a precipitant, non-additive fixative used for demonstration of enzymes, performed at refrigerator temperature, and used for brain tissue when rabies stains are needed?

• A. Acetone
• B. Methyl Alcohol
• C. Ethyl Alcohol
• D. Carnoy's Solution

Answer: (A)

Acetone works by precipitating proteins rather than forming cross-links, so it preserves enzymatic activity while fixing tissue. Because it fixes by dehydration at cold temperatures, it minimizes diffusion and helps keep enzymes active long enough to demonstrate activity in histochemical tests. In brain tissue, when rabies stains are needed, cold acetone fixation is preferred because it preserves antigenicity well enough for antibody-based detection, allowing reliable rabies antigen staining.

Methyl alcohol and ethyl alcohol are also precipitating fixatives, but they can be harsher on tissues or alter antigenicity or enzyme retention in some contexts. Carnoy's solution is a stronger, non-additive fixative that can cause more distortion and is less suitable for preserving enzymatic activity and rabies antigenicity in brain tissue.