This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'An undifferentiated malignant tumor on immunohistochemical stain shows cytoplasmic positivity of most of the tumor cells for cytokeratin. The most probable diagnosis of the tumor is \nSelect and explain. \nA. Carcinoma, B. Lymphoma, C. Sarcoma, D. Malignant melanoma', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Immunohistochemistry is used for making categorization of undifferentiated tumors. The important examples include: Cytokeratin (carcinoma), Desmin (Leiomyoma and Rhabdomyosarcoma) and vimentin (Sarcomas).', 'role': 'assistant'}]