## **Core Concept**
The question tests knowledge of protozoan parasites, specifically focusing on the characteristic presence of flagella in amoebas. Amoebas are typically non-motile or move using pseudopodia, but certain species exhibit flagellated forms.

## **Why the Correct Answer is Right**
The correct answer, *Naegleria*, is a genus of amoebas that exhibits a flagellated stage. *Naegleria* species, such as *Naegleria fowleri*, are known to cause primary amoebic meningoencephalitis. In their life cycle, *Naegleria* can transform from an amoeboid form to a flagellated form, which aids in their motility and pathogenicity. This transformation involves the expression of flagellin proteins and the assembly of flagella, enabling the amoeba to move more effectively.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Entamoeba* species, such as *Entamoeba histolytica*, are amoebas that primarily cause amoebic dysentery and liver abscesses. They do not exhibit a flagellated stage in their life cycle; instead, they move using pseudopodia.
- **Option B:** *Acanthamoeba* species are known for causing granulomatous amoebic encephalitis and are often associated with amoebic keratitis in contact lens wearers. Like *Entamoeba*, *Acanthamoeba* does not have a flagellated stage.
- **Option D:** *Giardia* is actually a flagellated protozoan but is not classified as an amoeba; it belongs to the class Giardiidae. *Giardia lamblia* causes giardiasis and has flagella for motility, but it does not fit the classification as an amoeba.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that *Naegleria fowleri* causes primary amoebic meningoencephalitis, a rare but often fatal disease. It is associated with warm freshwater environments, and infection typically occurs through nasal exposure to contaminated water. Rapid diagnosis and treatment are critical.

## **Correct Answer:** .