Answer: Cyclospora cayetanensis oocysts
Many of you correctly commented that the differential includes the oocysts of both Cryptosporidium spp. and Cyclospora cayetanensis. Both will stain with either the modified safranin stain or modified acid fast stain (the former tends to give better results, although the latter is more widely used). Therefore, it is essential to measure the oocysts to differentiate between these 2 organisms. The oocysts of Cryptosporidium spp. are 4-6 microns in diameter, while the oocysts of Cyclospora cayetanensis are 8-10 microns in diameter.