An HIV-infected patient with eight days of diarrhea has pink-stained round structures about 4 μm in diameter in acid-fast stool smears. The most likely pathogen is

• A. Blastocystis hominis
• B. Cryptosporidium sp.
• C. Cyclospora sp.
• D. Microsporidium

Answer: (B)

Acid-fast, pink-stained oocysts around 4 μm in the stool of an HIV-infected patient with diarrhea point most strongly to Cryptosporidium species. These oocysts are small, spherical, and reliably acid-fast, staining bright pink with the standard stains used for stool smears. In immunocompromised hosts, Cryptosporidium causes profuse, watery diarrhea that can be persistent for weeks, so this age-and-stain pattern fits the clinical scenario well.

Cyclospora oocysts are acid-fast too but larger, about 8–10 μm, so the 4 μm size helps distinguish them from Cryptosporidium. Blastocystis hominis is not typically described as acid-fast and has a different, more variable appearance. Microsporidium are much smaller and require special stains or techniques to visualize, rather than the classic acid-fast oocysts seen here.