How do Cryptosporidium oocysts typically appear on a modified acid-fast stain?

• A. Pink/red oocysts against a blue background
• B. Blue oocysts against a red background
• C. Purple oocysts on a green background
• D. Clear, unstained oocysts

Answer: (A)

The concept being tested is how modified acid-fast staining reveals Cryptosporidium oocysts based on their acid-fast properties. In this stain, the primary dye (carbol fuchsin) is retained by acid-fast organisms even after a decolorization step, so the oocysts take up a pink to red color. A counterstain then colors the surrounding background blue, making the pink/red oocysts stand out clearly. This creates the characteristic appearance of pink/red oocysts against a blue background. Other color schemes don’t match how the stain differentiates acid-fast organisms from the background, and would not reflect the positive, bright-pink/red staining of Cryptosporidium oocysts on this protocol.