What is the likely cause of diarrhea in a patient whose modified acid-fast stained fecal smear reveals round structures with variable staining?

The presence of round structures with variable staining in a modified acid-fast stained fecal smear strongly suggests the presence of a specific pathogen associated with gastrointestinal illness. Cyclospora cayetanensis is a protozoan parasite that often causes diarrhea and is known to be detected through modified acid-fast staining techniques.

Cyclospora exhibits a unique characteristic where its oocysts can display variability in staining intensity due to their thick walls and refractile appearance, which aligns with what would be observed in the fecal smear described. This pathogen is known to cause outbreaks of diarrheal illness, especially in tropical and subtropical regions, and can often be linked to contaminated food or water, highlighting its clinical relevance.

In contrast, while Blastocystis hominis can also be present in fecal samples, it does not typically exhibit the staining characteristics seen with Cyclospora in modified acid-fast stains. Polymorphonuclear leukocytes would indicate an inflammatory response and are not the causative organism itself, and large yeast cells would not appear as round structures with variable staining in this context. Hence, the identification of the round structures with variable staining firmly points towards Cyclospora cayetanensis as the likely cause of diarrhea in the patient.