**Core Concept**
Entamoeba histolytica is a protozoan parasite that causes amoebiasis, a disease characterized by intestinal infection, colitis, and extraintestinal complications. The diagnosis of E. histolytica is primarily based on the microscopic examination of stool samples, where the presence of characteristic cysts or trophozoites is determined.

**Why the Correct Answer is Right**
The most distinctive feature of pathogenic E. histolytica on fresh stool examination is the presence of **Ishihara bodies** or **pseudochromatin bodies**. These are basophilic, rounded inclusions within the cytoplasm of the trophozoites, which are formed by the accumulation of chromatin material. The presence of Ishihara bodies is a reliable indicator of pathogenic E. histolytica, as they are not seen in non-pathogenic amoebae, such as E. dispar.

**Why Each Wrong Option is Incorrect**
* **Option A:** Vacuolated appearance is not a distinctive feature of pathogenic E. histolytica, as it can be seen in both pathogenic and non-pathogenic strains.
* **Option B:** The presence of a clear zone around the nucleus is characteristic of E. dispar, not pathogenic E. histolytica.
* **Option D:** The absence of a clear zone around the nucleus is not a reliable indicator of pathogenic E. histolytica, as it can be seen in both pathogenic and non-pathogenic strains.

**Clinical Pearl / High-Yield Fact**
To differentiate pathogenic E. histolytica from non-pathogenic amoebae, it is essential to examine fresh stool samples and look for the presence of Ishihara bodies or pseudochromatin bodies. This can be a critical step in diagnosing amoebiasis and guiding appropriate treatment.

**Correct Answer: C. Ishihara bodies**