The Bauer-Faulen stain is a modification of which classic staining reaction?

• A. Feulgen reaction
• B. Gram reaction
• C. PAS reaction
• D. Silver stain

Answer: (A)

The Bauer-Faulen stain is a modification of the Feulgen reaction, which is the classic DNA-specific histochemical stain. In the Feulgen reaction, acid hydrolysis of DNA exposes aldehyde groups on the deoxyribose; these aldehydes react with Schiff’s reagent to yield a magenta color that marks nuclei and chromatin. The Bauer-Faulen variation tweaks the protocol to optimize staining of DNA-containing structures in certain tissues or preparations, but it still relies on the same principle and targets DNA. The other options are different staining categories (Gram for bacterial walls, PAS for carbohydrates, Silver stain for various structures) and do not describe a modification of the DNA-specific Feulgen reaction.