A Masson stain has been requested and the supply of phosphomolybdic acid has been depleted. What is the best action?

• A. Substitute phosphotungstic acid
• B. Use distilled water only
• C. Increase staining time
• D. Use a different fixative

Answer: (A)

Masson trichrome relies on a differentiating step with phosphomolybdic acid or phosphotungstic acid to separate muscle from collagen so the collagen component can be stained distinctly. When the phosphomolybdic acid supply is depleted, substituting phosphotungstic acid is the best action because it performs the same differentiating role and preserves the intended staining pattern for collagen. Using distilled water won’t replace the differentiator and won’t restore the distinct contrast; increasing staining time won’t compensate for the missing differentiator, and changing the fixative would alter tissue chemistry and staining behavior. Use phosphotungstic acid per protocol, with any necessary concentration/time adjustments as specified.