Answer: Cryptosporidium species.
Lee suggested that this is the newly recognized species, Cryptosporidium enterprisium.
Florida Fan rightly points out that, although considered the gold standard test for Cryptosporidium, the DFA may not detect all species that infect humans. Therefore, another test that can be used to confirm the identification of spherical, 4-6 micron diameter objects on wet prep, would be an Acid Fast stain. As the new molecular multiplex panels become commercially available, they also may be used as an initial or confirmatory test.