Answer: Giardia duodenalis (a.k.a. G. lamblia, G. intestinalis) trophozoites.
This was a very impressive case given the large number of trophozoites present. Tomáš Macháček asked how often this number of trophozoites is
found in stool compared with cysts. In my experience (and in the
experience of the Florida Fan who donated this case), this amount
of trophozoites is very uncommon - typically seen only in duodenal