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
[{'content': 'Perforin - \nSelect and explain. \nA. Cytotoxic T cell, B. NK cell, C. Plasma cell, D. Monocyte', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Cytotoxic T Lymphocyte-Mediated Apoptosis\n\nCytotoxic T lymphocytes (CTLs) recognize foreign antigens\xa0presented on the surface of infected host cells,\nUpon activation,\xa0CTLs secrete perforin, a transmembrane pore-forming molecule, which promotes entry\xa0of the CTL granule serine proteases called granzymes.\nGranzymes cleave proteins at aspartate residues and thus activate\xa0a variety of cellular caspases. In this\xa0way, the CTL kills target cells by directly inducing the effector phase of apoptosis.', 'role': 'assistant'}]