Answer: Trophozoites and Cysts of Acanthamoeba species/Balamuthia mandrillaris
A multiplex PCR for free-living amebae was also performed on this tissue, which was positive for Acanthamoeba sp. DNA.
Many of you noted that the organism morphology is characteristic for Acanthamoeba/Balamuthia (it is nearly impossible to tell the 2 apart by light microscopy, so I group them together) but mentioned that the skin is an unusual location for these organisms. It is correct that we don't usually see the free-living amebae in the skin. However, if you take a look at the CDC life cycle of Acanthamoeba and B. mandrillaris, you'll note that one port of entry to the blood stream (and eventually the brain) is broken skin. Therefore, it is essential to identify amebae in skin biopsies so that treatment can be implemented before dissemination occurs. In this case, note how the organisms are clustered around blood vessels, indicating their potential for hematogenous dissemination! Fortunately this patient did not have evidence of brain involvement.