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
A 67 year male smoker presents with haemoptysis and cough. Brochoscopic biopsy revealed undifferented tumour. The immunohistochemical marker that can be most helpful is: 
Select and explain. 
A. Calretinin, B. Vimentin, C. Cytokeratin, D. GGT
C: Cytokeratin
Answer: C: Cytokeratin 
Explanation: Cytokeratin