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 hemoptysis and cough. Bronchoscopic biopsy revealed an undifferentiated tumor. 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: None