The phenomenon that occurs when a single pure dye stains chromotropic tissue in a color that differs from that of the dye solution due to polymerization.

• A. Metachromasia
• B. Orthochromasia
• C. Counterstain
• D. Accentuator

Answer: (A)

Metachromasia is the phenomenon where a pure dye stains tissue a different color from the dye solution because the dye polymers or aggregates form within the tissue. Some dyes, notably toluidine blue, interact with tissue components such as sulfated glycosaminoglycans or mast cell granules, causing the dye to polymerize locally and shift its light absorption. As a result, the tissue may appear purple or red even though the dye solution is blue. This color change through dye polymerization is what distinguishes metachromasia from staining that simply matches the dye color (orthochromasia), from using a separate counterstain to provide contrast, or from using an accelerator to intensify staining.