Answer: Naegleria fowlerii trophozoites
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The key to the identification is recognizing the small nuclei with large karyosomes in the trophozoites. Note also the classic bubbly cytoplasm. These are characteristic features of the free-living amoebae. (CLICK ON IMAGE TO ENLARGE)
The differential diagnosis includes the other free-living amoebae Acanthamoeba spp. and Balamuthia mandrillaris. However, these 2 amoebae are typically seen within brain parenchyma (instead of the CSF) and also produce characteristic cysts (the dormant stage). The history also fits best for infection with N. fowleri, given the rapid state of deterioration in an otherwise healthy boy. Infection with the other two free-living amoebae is typically in immunocompromised adults, and has a subacute or chronic presentation. Of note, N. fowleri does not produce cysts in humans - only the trophozoite form is seen.