An undifferentiated malignant tumour on immunohistochemical stain shows cytoplasmic positivity of most of the tumor cells for cytokeratin. The most probable tumor is:

**Core Concept**
The question is testing the ability to identify a malignant tumor based on immunohistochemical staining patterns, specifically the expression of cytokeratin. Cytokeratin is a marker for epithelial cells, and its presence in a tumor suggests an epithelial origin.

**Why the Correct Answer is Right**
The presence of cytoplasmic positivity for cytokeratin in most tumor cells suggests that the tumor is of epithelial origin. Cytokeratins are intermediate filaments found in the cytoplasm of epithelial cells, and their expression is a characteristic feature of carcinomas. The tumor is likely a carcinoma, which is a malignant tumor of epithelial origin. The specific type of carcinoma cannot be determined solely based on cytokeratin expression, but it can help to narrow down the differential diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too broad. While it is true that some tumors may express cytokeratin, the presence of cytokeratin alone is not sufficient to diagnose a specific type of tumor.

**Option B:** This option is incorrect because it is not specific. Many types of carcinomas may express cytokeratin, and the presence of cytokeratin alone is not sufficient to diagnose a specific type of carcinoma.

**Option C:** This option is incorrect because it is not relevant. The presence of cytokeratin is not a characteristic feature of sarcomas, which are malignant tumors of mesenchymal origin.

**Clinical Pearl / High-Yield Fact**
When interpreting immunohistochemical stains, it is essential to consider the pattern and intensity of staining, as well as the specific markers being expressed. In this case, the presence of cytoplasmic positivity for cytokeratin in most tumor cells suggests an epithelial origin, but further testing may be needed to determine the specific type of carcinoma.

**Correct Answer: C. Carcinoma.**

✓ Correct Answer: C. Carcinoma