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
Which amongst the following a carinoma- 
Select and explain. 
A. Cytokeratin, B. Vimentin, C. Calretinin, D. CD45
A: Cytokeratin
Answer: A: Cytokeratin 
Explanation: Cytokeratins are proteins of keratin-containing intermediate filaments found in the intracytoplasmic cytoskeleton of epithelial tissue. The term "cytokeratin" began to be used in the late 1970s (for example, see "Intermediate-sized filaments of human endothelial cells" by Franke, Schmid, Osborn and Weber when the protein subunits of keratin intermediate filaments inside cells were first being identified and characterized.