## **Core Concept**
Eosinophilic meningoencephalitis is a condition characterized by the presence of eosinophils in the cerebrospinal fluid (CSF), often associated with parasitic infections or other specific etiologies. This condition reflects an immune response that involves eosinophils, which are typically elevated in parasitic infections or allergic reactions. The presence of eosinophils in the CSF is a critical diagnostic clue.

## **Why the Correct Answer is Right**
The correct answer, **Angiostrongylus cantonensis**, is a parasitic nematode known to cause eosinophilic meningoencephalitis. This parasite, also known as the rat lungworm, typically infects rats but can accidentally infect humans, leading to eosinophilic meningitis or meningoencephalitis. The infection occurs when humans ingest food or water contaminated with the third-stage larvae of the parasite. The larvae then migrate to the central nervous system (CNS), causing inflammation and an eosinophilic response.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain protozoa and helminths can cause CNS infections, **Toxoplasma gondii** primarily causes toxoplasmosis, which leads to encephalitis, particularly in immunocompromised individuals, but it is not characteristically associated with eosinophilic meningoencephalitis.
- **Option B:** **Naegleria fowleri** causes primary amebic meningoencephalitis, a condition that presents acutely with a high mortality rate. It is not associated with eosinophilia in the CSF.
- **Option D:** **Acantamoeba** species can cause granulomatous amebic encephalitis, primarily in immunocompromised patients, and is not characteristically linked with eosinophilic meningoencephalitis.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that eosinophilic meningoencephalitis should prompt consideration of parasitic infections, particularly in travelers or individuals with relevant exposure histories. **Angiostrongylus cantonensis** infection is a notable cause of eosinophilic meningitis worldwide, especially in regions where the parasite is endemic.

## **Correct Answer:** C. Angiostrongylus cantonensis.