Corynebacterium diphtheriae

Which toxin produced by Corynebacterium diphtheriae is responsible for the characteristic pseudomembrane formation in diphtheria?

Which of the following is the biochemical basis for the pathogenic effect of diphtheria toxin on host cells?

Regarding the virulence factors and pathogenesis of Corynebacterium diphtheriae, all of the following statements are correct EXCEPT:

A 4-year-old child from Delhi presents with a 5-day history of sore throat and fever. Throat culture on Loeffler's serum medium shows grey-white colonies. Gram stain reveals Gram-positive bacilli arranged in a Chinese letter pattern. Elek test is positive. The child has already received 40,000 units of diphtheria antitoxin 24 hours ago and is on penicillin V. Today, the child develops acute onset of weakness in both legs with inability to walk, and mild ptosis. What is the most appropriate next step?

A 7-year-old unvaccinated boy from rural Maharashtra presents with a 3-day history of sore throat, low-grade fever (38.2°C), and progressive difficulty swallowing. On examination, he has a thick, greyish-white pseudomembrane covering the pharynx and tonsils that bleeds when attempts are made to remove it. Cervical lymphadenopathy is marked, giving a "bull neck" appearance. A throat swab is sent for culture. Which organism is most likely responsible, and what is the pathogenic mechanism of systemic toxicity?