The answer to Case 410 is amebic keratitis due to Acanthamoeba sp.
As several of you mentioned, this organism can be identified by the presence of both trophozoites and cysts in the patient's specimen, as well as the characteristic source (ocular/contact lens) and exposure history (swimming in fresh water). Both Acanthamoeba spp. and Balamuthia mandrillaris form cysts and trophozoites in humans, whereas Naegleria fowleri only forms trophozoites (and rarely flagellated forms in cerebrospinal fluid), and therefore the latter can be excluded by the presence of the cyst forms. Of the two organisms that do form cysts in humans, only Acanthamoeba species are known to cause ocular infections - primarily in patients who wear contact lenses, thus giving us our final diagnosis.