Answer: Blastocystis hominis
This was a very unusual and interesting case which generated a lot of great discussion. Thank you for the comments! Blaine (who donated this case) and I both believe that this case represents contamination/colonization of pre-existing fasciitis with B. hominis in a patient with Fournier's gangrene, rather than primary infection with this parasite. Fournier's gangrene is an extensive and rapidly-progressive polymicrobial fasciitis often seen in the setting of impaired host response (e.g. diabetes) and is usually precipitated by local trauma. The necrotizing process commonly originates from the anorectal tract, which might explain how B. hominis came to be involved.
This case gives us a rare opportunity to see the classic vacuolar forms of B. hominis in H&E-stained tissue sections. They have a very similar appearance to what we see in stool exams, with a large central vacuole and peripheral nuclei.