A 6-year-old unvaccinated child from rural India presents with fever, sore throat, and a characteristic gray-white adherent pseudomembrane over the tonsils and pharynx that bleeds on removal. Throat swab culture on Loeffler medium confirms Corynebacterium diphtheriae. The structure marked **B** in the diagram is the component of diphtheria toxin responsible for the pathogenic effect. Which of the following best describes the molecular mechanism by which this structure causes cell death?