A 30-year-old man with AIDS and a CD4 count below 50 cells/mm3 has profuse non-bloody watery diarrhoea unresponsive to antibiotics. Modified Ziehl-Neelsen staining of stool shows pink-staining spherical structures measuring 4 to 6 micrometres. These structures represent:
- A Acid-fast oocysts of Cryptosporidium species ✓
- B Operculated eggs of Clonorchis sinensis
- C Acid-fast sporulating oocysts of Isospora belli measuring 20 to 30 micrometres
- D Spherical cysts of Entamoeba histolytica with four nuclei
Correct answer: A. Acid-fast oocysts of Cryptosporidium species
Explanation
Cryptosporidium parvum oocysts are small (4 to 6 micrometres), round, and stain pink-red with modified acid-fast stains, appearing as unstained ghosts against the green background. Their tiny size separates them from Cystoisospora belli, whose acid-fast oocysts are much larger (20 to 30 micrometres) and elongated. Operculated eggs belong to trematodes, and Entamoeba cysts are not acid fast, so the remaining options fail both the size and staining criteria.
Reference: Paniker's Textbook of Medical Parasitology, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.