Which motile amoebae are most likely responsible for meningitis in a child who swam in a warm-water spring?

Naegleria fowleri is the correct answer because this free-living amoeba is known to cause primary amoebic meningoencephalitis (PAM), a rare but severe form of meningitis, particularly in individuals who have been exposed to warm freshwater environments, such as hot springs or lakes. The pathogenicity of Naegleria fowleri is linked to its ability to enter the human body through the nasal mucosa during activities like swimming, leading to rapid and often fatal infections.

The other amoebae listed are generally not associated with meningitis. Iodamoeba bütschlii and Endolimax nana are typically considered commensals of the human intestine and are not linked to neurological disease. Dientamoeba fragilis is also primarily an intestinal parasite and is not known to cause meningoencephalitis. Thus, Naegleria fowleri's presence in warm freshwater environments makes it the primary concern in cases of meningitis following swimming in such locations.