The most important step in regressive hematoxylin staining is what process?

• A. Differentiation in acid-alcohol
• B. Bluing
• C. Differentiation in alkaline solution
• D. Rinsing

Answer: (A)

Regressive hematoxylin staining relies on overstaining the tissue and then precisely removing the excess dye so that only the nuclei retain strong, specific color. The differentiation in acid-alcohol does this trimming work: the weaker, non-nuclear staining is dissolved away while the hematoxylin bound to nuclear DNA remains, giving crisp nuclear detail with low background. This step is what sets the final contrast and sharpness of the nuclei, and it must be timed carefully—too long and the nuclei fade; too short and background staining persists.

Bluing follows to convert the hematoxylin to its blue form and stabilize the color, but it’s the acid-alcohol differentiation that defines the effectiveness of the nuclear staining in the regressive method. Rinsing is a general wash, and differentiation in alkaline solution is not the technique used here, as it would alter the staining pattern differently rather than selectively remove the overstain.