Final answer:
The diphtheria toxin is an A-B exotoxin made up of two domains: the catalytic domain for inhibiting protein synthesis and the binding domain for attaching to specific cell surface receptors.Hence, the correct answer is option A.
Step-by-step explanation:
The diphtheria toxin is an example of an A-B exotoxin, which consists of two domains: the catalytic domain (A subunit) and the binding domain (B subunit).
The catalytic domain of the diphtheria toxin is responsible for inhibiting protein synthesis in the host cell. It does this by transferring an ADP-ribose onto an elongation-factor protein (EF-2) that is essential for protein synthesis.
The binding domain of the diphtheria toxin mediates the initial attachment of the toxin to specific cell surface receptors, allowing the toxin to enter the host cell via endocytosis.