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 56-year-old chronic smoker, mass in bronchus resected. What is the possible marker- 
Select and explain. 
A. Cytokeratin, B. Vimentin, C. Epithelial membrane Cadherin, D. Leukocyte
A: Cytokeratin
Answer: A: Cytokeratin 
Explanation: Squamous cell carcinoma is most commonly found in men and is closely correlated with a smoking histoty.
Histologically, it is characterized by keratinization and intracellular bridges.
Cytokeratin is a marker for keratinizing cells.