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. Bronchoscopic biopsy revealed undifferentiated 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: Ans is 'c' i.e., Cytokeratin o Mass in bronchus in a old chronic smoker suggest bronchogenic carcinoma. The marker of carcinoma, on immunohistochemistry, is cytokeratin.