What is the mechanism by which Corynebacterium diphtheriae exotoxin acts?

The mechanism by which Corynebacterium diphtheriae exotoxin operates involves the ADP-ribosylation of elongation factor 2 (EF-2). This is a crucial step in protein synthesis, as EF-2 is essential for the translocation of the ribosome along the mRNA during translation. When the diphtheria toxin ADP-ribosylates EF-2, it effectively inactivates this factor, leading to a halt in protein synthesis within the affected cells. The consequence is cellular dysfunction and eventually cell death, which can have severe effects on tissues, especially in the heart and nervous system, contributing to the clinical manifestations of diphtheria.

The other options relate to different mechanisms of action for various toxins or drugs: inhibiting DNA synthesis pertains to actions of certain antibiotics or antineoplastic agents, denaturing proteins is a general description that can apply to heat or certain denaturing agents, and inhibiting RNA polymerase is characteristic of different classes of antibiotics but not the specific action of diphtheria toxin. Thus, the unique action of ADP-ribosylation of EF-2 clearly defines the correct answer for this question regarding the diphtheria exotoxin.