An undifferentiated malignant tumour on immunohistochemical stain shows cytoplasmic positivity of most of the tumor cells for cytokeratin. The most probable diagnosis of the tumor is?
**Core Concept**
The question assesses the ability to identify a malignant tumor based on its immunohistochemical characteristics, specifically the expression of cytokeratin. Cytokeratins are a family of intermediate filament proteins that are expressed in epithelial cells, making them useful markers for diagnosing epithelial-derived tumors.
**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 typically expressed in epithelial cells, and their expression in a tumor is a strong indication that the tumor is derived from epithelial tissue. The undifferentiated nature of the tumor indicates that it may be a carcinoma, which is a malignant tumor of epithelial origin.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cytokeratin expression is not typically associated with lymphomas, which are tumors of lymphoid origin.
**Option B:** This option is incorrect because while cytokeratin expression can be seen in some types of sarcomas, it is not a characteristic feature of these tumors.
**Option C:** This option is incorrect because melanomas are tumors of melanocytic origin, and cytokeratin expression is not typically associated with these tumors.
**Clinical Pearl / High-Yield Fact**
When interpreting immunohistochemical stains, it's essential to consider the tumor's morphology and the specific markers being tested. Cytokeratin expression is a useful marker for identifying epithelial-derived tumors, but it's not a specific marker for any particular type of carcinoma.
**Correct Answer: C. Melanoma.