This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pathology, Subject name: Pathology
A 67 year nude smoker presents with haemoptysis and cough. Bronchoscopic biopsy revealed un-differentiated 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: TEXT BOOK OF PATHOLOGY HARSHMOHAN 6TH EDITION PG NO 16 Epithelial tumors are stained by pankeratin or cytokeratin (high or low MW) <\p>