This correct answer is C, Cyclospora cayetanensis.  The images provided for this case show the usefulness of alternative methods for identifying this coccidian parasite.  The diagnostic features included are:

• Oocyst walls of C. cayetanensis autofluoresce under ultraviolet light if a microscope is equipped with the necessary equipment to allow such viewing.
• Oocysts of C. cayetanensis appear as nonrefractile spheres that contain undifferentiated cytoplasm that can appear as refractile globules when viewed using differentiated interference contrast (DIC) microscopy.
• Oocysts are within the size range consistent with C. cayetanensis, 8 to 10 µm.

The modified Kinyoun’s acid-fast stain is the most common method used for the detection of C. cayetanensis; it also provides a permanent record.  However, because C. cayetanensis stains variably, some oocysts may stain dark red or pink, while others may not stain at all.