How is the technique shown in the image best described?

• A. regressive
• B. progressive
• C. additive
• D. differential

Answer: (A)

Regressive staining describes overstaining followed by a controlled decolorization to sharpen contrast between structures. In this approach, you apply the stain more than needed and then remove the excess with a solvent. Structures with high affinity retain more stain after decolorization, while those with lower affinity lose color, creating a differential, two-tone appearance. This method is used to reveal differences more clearly, as in Gram staining where bacteria that retain the primary stain appear purple and others take up the counterstain. The image likely shows that exact overstain-then-decolorize pattern, which is characteristic of regressive staining. Additive would produce uniform color with no selective decolorization, progressive would intensify color without a decolorization step, and differential is a broad outcome term, not the specific overstain-and-decolorize sequence.