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': 'A 67 year male smoker presents with haemoptysis and cough. Brochoscopic biopsy revealed undifferented tumour. The immunohistochemical marker that can be most helpful is: \nSelect and explain. \nA. Calretinin, B. Vimentin, C. Cytokeratin, D. GGT', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Cytokeratin', 'role': 'assistant'}]