Final answer:
Classical C3 convertase is C4b2a, crucial for the immune system's complement pathway. It splits C3 into C3a and C3b, leading to the formation of C5 convertase, and further activation of the complement system towards the MAC. The correct option is A.
Step-by-step explanation:
The classical C3 convertase is a complex of proteins that plays a pivotal role in the complement system, part of the immune response. The correct answer to the question 'Classical C3 convertase is' is a. C4b2a. The C3 convertase enzyme is crucial as it cleaves the C3 molecule into two fragments: C3a and C3b.
After C3 is split, the larger fragment C3b can bind back to C3 convertase to form the C5 convertase, which further progresses the complement cascade towards the formation of the membrane-attack complex (MAC).
The classical pathway is triggered when C1 binds to an antigen-antibody complex on the surface of a pathogen, which leads to the activation and splitting of complement components C2 and C4, thus forming the C3 convertase, C4b2a. In contrast, the alternate pathway activates C3 spontaneously without the need for antibodies.
Regarding the complement activation pathway that is triggered by the binding of an acute-phase protein to a pathogen, the correct answer is A. classical.