## **Core Concept**
The question tests knowledge about the characteristics of *Entamoeba histolytica* cysts, which are a key diagnostic feature in amoebiasis. *E. histolytica* is a protozoan parasite that causes amoebic dysentery and liver abscesses. The cyst form is crucial for its survival outside the host and for transmission.

## **Why the Correct Answer is Right**
The correct answer, ., refers to the fact that *E. histolytica* cysts characteristically contain four nuclei, not more or less, when mature. This is a key identifying feature. The presence of four nuclei is a diagnostic criterion for differentiating *E. histolytica* from other amoebas. The cysts are usually 10-20 μm in diameter and have a thick, refractile wall.

## **Why Each Wrong Option is Incorrect**
- **Option A:** . *E. histolytica* cysts indeed have a thick wall which helps in their survival outside the host.
- **Option B:** . The size range of *E. histolytica* cysts is typically 10-20 μm, making this statement correct about the cysts.
- **Option D:** . *E. histolytica* cysts do contain chromatoid bodies, but they are usually not seen in mature cysts; they are more characteristic of young cysts or other forms of the parasite.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the presence of four nuclei in a cyst is highly suggestive of *E. histolytica*. This feature is critical for the diagnosis of amoebiasis. Additionally, it's essential to differentiate *E. histolytica* from *Entamoeba coli*, which can have up to eight nuclei in its cysts.

## **Correct Answer:** . Chromatoid bodies are not typically seen in mature cysts.