MCQ of the Day: 14 November 2026

Answer: C. Naegleria fowleri. This is primary amoebic meningoencephalitis (PAM). Infection follows entry of warm fresh water into the nose during swimming or diving; trophozoites invade the olfactory mucosa and cross the cribriform plate, which explains the early change in smell and taste. The illness is acute and fulminant, and the CSF resembles bacterial meningitis (neutrophils, low glucose, high protein) but the Gram stain is negative. The key bedside test is a wet mount of fresh, warm (never refrigerated) CSF, which shows motile trophozoites with eruptive lobopodia. N. fowleri is an amoeboflagellate: in distilled water the trophozoites transform into a temporary pear-shaped form with two flagella (the enflagellation test). Confirmation is by real-time PCR. Mortality exceeds 90%.

A. Acanthamoeba causes granulomatous amoebic encephalitis, a subacute illness, mostly in immunocompromised people, and keratitis in contact-lens wearers; it has no flagellate stage.

B. Entamoeba histolytica reaches the brain rarely, by blood-borne spread from liver abscess; it has no flagellate stage.

D. Balamuthia mandrillaris also causes chronic granulomatous encephalitis, often after skin lesions, and does not form flagella.