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: Microbiology
[{'content': 'Which of the following best denotes classical complement pathway activation in immune inflammatory condition? \nSelect and explain. \nA. C2, C4 and C3 decreased, B. C2 and C4 normal, C3 is decreased, C. C3 normal and C2, C4 decreased, D. C2, C4, C3 all are elevated', 'role': 'user'}, {'content': 'Answer: A \nExplanation: In the classic pathway, antigen-antibody complexes activate C1 to form a protease, which cleaves C2 and C4 to form a C4b,2b complex. The latter is C3 convease, which cleaves C3 molecules into two fragments, C3a and C3b. Because the immune complex activation involves the classical components, C4, C2, and C3, their serum concentration decreases. Ref: Levinson W. (2012). Chapter 63. Complement. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.', 'role': 'assistant'}]