If phosphomolybdic acid is unavailable for a Masson stain, which reagent is an appropriate substitute?

• A. Phosphotungstic acid
• B. Molybdic acid
• C. Sodium thiosulfate
• D. Hematoxylin

Answer: (A)

In Masson’s trichrome, a differentiating reagent is used after the nuclear stain to prepare tissue so that the collagen-dye (blue) binds selectively. Phosphomolybdic acid and phosphotungstic acid perform this role by modifying tissue components in a way that allows collagen to take up the blue dye while other structures do not. If phosphomolybdic acid isn’t available, phosphotungstic acid is the appropriate substitute because it gives the same differentiating effect and works with the same staining sequence and dyes. Using the substitute preserves the intended separation of colors and makes collagen appear blue with the final stain. Molybdic acid on its own wouldn’t provide the same differentiating action in this protocol, and sodium thiosulfate is not used as the differentiator in Masson’s, while hematoxylin is the nuclear stain rather than a differentiating reagent.