Answer: Giardia intestinalis (a.k.a. G. lamblia, G. duodenalis)
Even though the morphology is a little indistinct in these histologic sections, the classic shape of the trophozoites can be identified. In some examples, the 2 nuclei and ventral sucking disk can also be seen (with a little imagination!). The nice thing about histologic sections is that you also get the opportunity to see the trophozoites from the lateral view and realize how flat they actually are. I tell my students that the curved thin shape of the trophozoites from the lateral view allows me to better understand their "falling leave motility."