Which stain has a critical time for hydrolysis?

• A. Feulgen
• B. Methyl violet
• C. Hematoxylin
• D. PAS

Answer: (A)

The key idea is that this stain relies on a tightly controlled acid hydrolysis step to reveal aldehyde groups on the sugar part of DNA, which the Schiff reagent then binds to for a magenta nuclear color. The duration of that hydrolysis is critical because it sets how many aldehyde sites are generated. If hydrolysis is too short, not enough aldehyde groups form and the nuclei stain weakly or not at all. If it’s too long, the DNA is over-hydrolyzed and the nuclear morphology can be damaged or the staining becomes non-specific. This precise timing ensures staining is specifically marking DNA in the nucleus, which is the defining feature of this method. Other stains don’t depend on this same hydrolysis step in the same way, so they don’t require this critical timing to produce their results.