Which staining technique is described as used for detecting carbohydrates in tissue?

• A. PAS
• B. Alcian blue pH 2.5
• C. Crystal violet
• D. Colloidal iron

Answer: (A)

Periodic acid–Schiff staining detects carbohydrates in tissue by oxidizing vicinal diols in sugars to aldehydes, which then react with Schiff reagent to produce a magenta color. This highlights a broad range of carbohydrate-containing substances such as glycogen, neutral mucopolysaccharides and mucosubstances, glycoproteins, and basement membranes (and fungal cell walls, which have polysaccharides). If you need to distinguish glycogen specifically, you can pre-digest with diastase; glycogen will be removed and the PAS reaction will fade for that component.

Alcian blue at pH 2.5 stains acidic mucopolysaccharides (acidic mucins) rather than all carbohydrates. Crystal violet is not a carbohydrate stain; it’s used as a general contrast stain (and in Gram staining contexts). Colloidal iron stains certain carbohydrate-containing substances but is more limited in scope. Thus, PAS is the best choice for detecting carbohydrates in tissue overall.