That was a tricky one! The object in the Image Challenge 22 is yeast.
The fictitious clinical history , the morphology and size of the microscopic image convinced most to report as oocyst of Cryptosporidium.
But a trained eye and closer observation of the object in the image could pick up the uniform staining (as if painted in pinkish red), that is totally uncharacteristic of Cryptosporidium.
The oocysts of Cryptosporidium typically stain non-uniform and have a pale centre owing to the presence of sausage shaped sporozoites (See the above image).
Although most yeasts are not acid-fast and take up the colour of the counterstain in the acid-fast procedures, occassionally strains of yeasts that are acid fast could be encountered, as in the Image Challenge. Without careful observation, they could be confused with Cryptosporidium oocysts. The differential characteristic between the two is that Cryptosporidium oocysts contain sporozoites whereas yeasts do not. So, while observing modified acid fast stained preparations, look carefully for the presence of pink sausage shaped sporozoites in the suspected oocysts. If you find sporozoites and pale centres and uneven staining character, you can make a positive identification of Cryptosporidium.
Reference: Laboratory diagnosis of intestinal Cryptosporidiosis,Course no 8008-C, US Department of Health and Human Services, CDC, Atlanta, USA
Image Courtesy: DPDx
Related Posts: Previous Image Challenges
Keywords: Cryptosporidiosis, Cryptospordium parvum, Coccidian parasites, diarrhea, Modified AFB stain, yeast, Cryptosporidium