**Core Concept**
Acute meningoencephalitis is a severe inflammatory condition affecting the meninges and brain parenchyma, often due to invasive or neurotropic pathogens. Among the listed organisms, *Nagleria* (specifically *N. fowleri*) is a free-living amoeba that causes primary amoebic meningoencephalitis (PAM), a rapidly progressive and fatal central nervous system infection.

**Why the Correct Answer is Right**
*Nagleria fowleri* is a thermophilic free-living amoeba found in warm freshwater. It enters the CNS via the nasal passages during swimming or exposure to contaminated water, then invades the brain and meninges. It causes acute, fulminant meningoencephalitis with characteristic symptoms like headache, fever, seizures, and altered mental status. The infection progresses rapidly, with a mortality rate of over 90%. The pathogenesis involves direct invasion of neural tissue and induction of severe inflammation.

**Why Each Wrong Option is Incorrect**
Option A: *E. histolytica* causes amoebic dysentery and liver abscesses, not meningoencephalitis. It does not invade the CNS.
Option C: *Giardia* causes gastrointestinal infections (e.g., giardiasis), with no known CNS involvement.
Option D: *E. coli* can cause urinary tract infections or sepsis, but not acute meningoencephalitis unless in rare cases of septicemia or meningitis (e.g., neonatal), not directly via CNS invasion.

**Clinical Pearl / High-Yield Fact**
PAM caused by *N. fowleri* is rare but life-threatening; it presents with sudden onset, headache, and seizures after exposure to warm water. Early recognition and treatment (e.g., with miltefosine or sodium phosphonate) are critical but ineffective due to rapid progression. Always consider PAM in patients with recent exposure to warm freshwater.

✓ Correct Answer: B. Nageleria