Which fixatives must be washed out with running water?

• A. Zenker and Helly
• B. Bouin and Carnoy
• C. Zenker and Bouin
• D. Helly and Carnoy

Answer: (A)

Mercury-containing fixatives require thorough washing with running water to remove residual mercuric chloride from the tissue. The mercuric ions are highly reactive and can continue to fix tissue if not flushed out, leading to artifacts and difficulty with staining. Zenker's fixative and Helly's fixative both use mercuric chloride as a key component, so after fixation the tissue must be rinsed with running tap water to dilute and remove these mercury salts. This reduces background staining, improves subsequent staining, and protects personnel from exposure.