This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiration, 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: Ans. (a) Cytokeratin(Ref: Robbins 9th/pg 715-717)Chronic smoker with mass in bronchus suggests a diagnosis of small ell Ca and squamous cell ca.Diagnosis for squamous cell Ca is Keratinization ("squamous pearls") Q on light microscopy, diagnosed by cytokeratin and/or Intercellular bridges Q on electron microscopy is diagnostic.