Which stains are used for identifying carbohydrates and mucosubstances in tissues?

• A. Hematoxylin and eosin
• B. PAS and PAS-D
• C. Masson's trichrome
• D. Ziehl-Neelsen

Answer: (B)

The key idea here is a stain that specifically highlights carbohydrates and mucosubstances in tissues. The Periodic acid–Schiff reaction makes glycogen, mucopolysaccharides, mucins, and related proteoglycans turn a magenta color by forming aldehyde groups in sugars that react with Schiff reagent. This makes PAS excellent for visualizing carbohydrate-rich substances and the basement membranes they’re part of.

Using diastase digestion before staining (PAS-D) helps distinguish glycogen from other carbohydrates. Diastase breaks down glycogen, so after digestion any remaining PAS positivity points to non-glycogen carbohydrates such as mucopolysaccharides and mucins. This differential approach is especially useful when you need to know whether a PAS-positive substance is glycogen or another mucosubstance.

Other stains listed don’t target carbohydrates or mucosubstances specifically: hematoxylin and eosin is general tissue morphology, Masson’s trichrome highlights collagen, and Ziehl-Neelsen detects acid-fast bacteria.