This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology Pa 1 (Immune Response, Antigen-Antibody Reactions, Hypersensitivity, Structure of Immune System, Immunodeficiency Disorders), Subject name: Microbiology
[{'content': 'Lysozyme is present in all except: \nSelect and explain. \nA. Lacrimal secretions, B. CSF, C. Saliva, D. Respiratory tract secretions', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Lysozyme is a heat-labile protein which acts as a muraminidase and hence has bactericidal activity. It is present in tears, saliva and all mucosal secretions, but is absent in CSF, sweat and urine', 'role': 'assistant'}]