This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neoplasia, Subject name: Pathology
How will you differentiate a mediastinal mass being a thymoma or acute lymphoblastic leukemia? 
Select and explain. 
A. Cytokeratin, B. CD1a, C. CD3, D. Tdt
A: Cytokeratin
Answer: A: Cytokeratin 
Explanation: Ans. A. CytokeratinA mediastinal mass has a differential of thymoma and T-cell ALL. T-cell ALL will have the markers TDT (lymphoblast), CD la and CD3. Thymoma has 2 components--epithelial and lymphoid components--so is positive for EMA, cytokeratin 7 and 20,CD57CD5,bcl-2, calretinin, vimentin, CD3,CD1a,CD20,CD99.