Which protozoan parasite is known for producing oocysts that are variably acid-fast and autofluoresce under UV light?

• A. Giardia lamblia
• B. Entamoeba histolytica
• C. Cyclospora cayetanensis
• D. Trichuris trichiura

Answer: (C)

Cyclospora cayetanensis is identified in stool by two distinctive features: its oocysts stain variably with acid-fast stains and they autofluoresce under UV light. This combination is characteristic and helps distinguish it from other intestinal parasites. The oocysts are about 8–10 micrometers in diameter and can show partial, inconsistent acid-fast staining with modified techniques, which is why the staining is described as variably acid-fast. When examined under a UV lamp, these oocysts emit a blue-green autofluorescence, a clue often used to confirm the organism in practice.

Other common parasites shown in stool do not exhibit this pair of traits. Giardia lamblia cysts are not acid-fast and do not autofluoresce under UV. Entamoeba histolytica cysts likewise lack acid-fast staining and autofluorescence. Trichuris trichiura, a helminth egg, is not an oocyst and does not display acid-fast staining or UV autofluorescence.

So, the presence of oocysts that are variably acid-fast and autofluoresce under UV light points to Cyclospora cayetanensis.