Which statement about the technique is true?

• A. It depends on the formation of aldehydes
• B. It uses glutaraldehyde as fixative
• C. It is best performed on paraffin-embedded sections
• D. It stains nucleic acids

Answer: (A)

The key idea is that this technique detects carbohydrates by first forming aldehyde groups on them, which then react with Schiff reagent to produce a magenta stain. Periodic acid oxidizes the vicinal diols in glycogen, glycoproteins, and other carbohydrate-rich substances to aldehydes, and those aldehyde sites are what the dye binds to. That aldehyde-formation step is the essential feature, so saying the technique depends on aldehydes is the best description. The other points aren’t defining: it’s not about using glutaraldehyde as a fixative, since PAS works with common fixatives like formalin and isn’t defined by the fixative; while it’s commonly performed on paraffin-embedded sections, that isn’t what makes the technique work; and it does not stain nucleic acids (nucleic acids are shown with different methods such as the Feulgen reaction).