A black precipitate on periodic acid-methenamine silver stained section after storage indicates which step was omitted?

• A. Sodium thiosulfate
• B. Dehydration
• C. Counterstain
• D. Rehydration

Answer: (A)

In this silver-staining method, after the tissue is impregnated with silver, a sodium thiosulfate step is used to dissolve and remove any unbound silver. This fixes the silver in place and prevents background deposition. If that thiosulfate step is skipped, residual silver remains in the tissue. Over time, during storage, this unused silver can continue to react and form dark, black deposits, giving a black precipitate on the slide. So a black precipitate after storage indicates that the sodium thiosulfate step was omitted. The other steps—dehydration, rehydration, and counterstaining—affect preparation and contrast but aren’t the cause of this post-storage artifact.