<s>[INST] context: Amongst the given options Naegleria and Acanthamoeba cause amoebic encephalitis.

“The prognosis of Naegleria encephalitis is uniformly poor, most patients die within a week” -------Harrison “Acanthamoeba encephalitis follows a more indolent course”.
--------Harrison

 So, amongst the given options Naegleria causes most fatal amoebic encephalitis.

Naegleria fowleri

 It is a free-living amoeba
 Exists in three forms - Amoeboid trophozoite, Biagellate trophozoite, Cyst (Never forms in the human body)
 Modes of transmission - Human primarily contact the infection by swimming in contaminated water.

The amoeboid trophozoite enters the human body through the nasal mucosa and migrates to the brain along the olfactory nerve and blood vessels.
Some infections result from inhalation of cysts.

 Clinical features (primary amoebic meningoencephalitis) Common in young adults and children.

Incubation period →2-15 days (5 days) A headache, high fever, nausea, vomiting, meningismus. Cranial nerve palsies —» 3rd, 4th and 6th. Most patients die within a week (95%).
N. fowleri also causes humidifier fever, a form of allergic alveolitis resulting from inhaling air from air-coolers.
 Diagnosis - Detection of motile amoeboid trophozoites in wet mounts of C.S.F.
 Treatment - High doses of amphotericin B and rifampicin.
Acanthamoeba - It is a free-living amoeba

 Acanthamoeba culbertsoni species is most common,

 Exists in two stages - Trophozoite, Cyst

 The appellate stage is absent
 Modes of transmission - Through contamination of traumatized skin or eye Inhalation. [/INST] question: A patient presents with a headache, high fever and meningismus. Within 3 days he becomes unconscious. Most probable causative agent | options: A: Naegleria fowleri B: Acanthamoeba castellanii C: Entamoeba histolytica D: Trypanosoma cruzi | answer: A </s>