Final answer:
The C3 convertase of the classical and lectin pathways is formed by the combination of complement components C4b and C2a, making the correct answer c. C4b2a.
Step-by-step explanation:
The enzyme complex known as C3 convertase plays a crucial role in the complement system, which is part of the immune response. In the context of the classical and lectin pathways, C3 convertase is formed by the combination of two complement components: C4b and C2a. Therefore, the correct answer to which molecule is the C3 convertase of the classical/lectin pathways is c. C4b2a.
When the C1 complex binds to an antigen-antibody complex on the surface of a pathogen in the classical pathway, it leads to the cleavage of C2 and C4 into two fragments each. The resulting fragments, C4b and C2a, then combine to form the C3 convertase of the classical pathway, C4b2a. In the lectin pathway, similar processes occur but the pathway is activated by an acute-phase protein, typically mannose-binding lectin (MBL), instead of an antibody. The molecule C4b2a also then cleaves C3 into two fragments, leading to the further progression of the complement cascade and eventually the formation of the C5 convertase when one of these fragments joins the C3 convertase.