Which two solutions form the high molecular weight complex in Gram's staining?

• A. Crystal Violet and Gram's Iodine
• B. Crystal Violet and Safranin
• C. Safranin and Gram's Iodine
• D. Methylene Blue and Iodine

Answer: (A)

In Gram staining, the high molecular weight complex is created when the primary stain (crystal violet) interacts with the mordant (Gram’s iodine). The iodine binds to crystal violet and forms a CV–I complex that is large and becomes trapped inside the thick peptidoglycan layer of Gram-positive cells. This complex withstands the decolorization step, so those cells stay purple, while Gram-negative cells lose the dye and take up the counterstain.

The other pairings don’t form that retained complex: the counterstain (safranin) is used after decolorization, not to build the large dye–mordant complex; methylene blue isn’t involved in forming the CV–I complex in Gram staining.