Which intestinal parasite has acid-fast staining that is variable and produces spores about 1-3 micrometers in diameter?

Which intestinal parasite has acid-fast staining that is variable and produces spores about 1-3 micrometers in diameter?

• A. Cyclospora cayetanensis
• B. Cryptosporidium parvum
• C. Isospora belli
• D. Microsporidium

Answer: (D)

The key clue is a tiny, variable acid-fast organism. Microsporidia form very small spores, about 1–3 micrometers in diameter, which is a distinctive size for intestinal parasites. Their staining with acid-fast methods can be inconsistent or weak, depending on the preparation, so the acid-fast reaction is variable rather than consistently positive. This combination helps separate it from others: Cyclospora cayetanensis has larger oocysts (around 8–10 μm) and while the acid-fast stain can vary, size argues against it; Cryptosporidium parvum produces oocysts about 4–6 μm and tends to stain acid-fast more consistently; Isospora belli has much larger oocysts (20–30 μm). Thus, the very small, variably acid-fast spores point to microsporidia.

The key clue is a tiny, variable acid-fast organism. Microsporidia form very small spores, about 1–3 micrometers in diameter, which is a distinctive size for intestinal parasites. Their staining with acid-fast methods can be inconsistent or weak, depending on the preparation, so the acid-fast reaction is variable rather than consistently positive.

This combination helps separate it from others: Cyclospora cayetanensis has larger oocysts (around 8–10 μm) and while the acid-fast stain can vary, size argues against it; Cryptosporidium parvum produces oocysts about 4–6 μm and tends to stain acid-fast more consistently; Isospora belli has much larger oocysts (20–30 μm). Thus, the very small, variably acid-fast spores point to microsporidia.