A 6-year-old unvaccinated child from rural India presents with sore throat, low-grade fever, and malaise for 3 days. On examination, the structure marked **A** in the diagram is noted — a grey-white adherent membrane on the tonsils extending to the pharynx. When gently scraped, the membrane bleeds. Which of the following best explains the pathophysiology of this clinical presentation?

Explanation

## Why option 1 is correct The grey-white adherent pseudomembrane on the tonsils and pharynx marked **A** is pathognomonic for diphtheria caused by *Corynebacterium diphtheriae*. The key pathophysiologic mechanism is that the diphtheria toxin inhibits elongation factor-2 (EF-2) via ADP-ribosylation, blocking protein synthesis and causing necrosis of pharyngeal tissue. This toxin-mediated mechanism is the hallmark of diphtheria and explains both the local pseudomembrane formation (fibrinous exudate from tissue necrosis) and systemic complications (myocarditis, neuropathy). The adherent nature of the membrane and bleeding when scraped are clinical hallmarks that distinguish diphtheria from other causes of pharyngitis. This is a toxin-mediated disease, not an invasive infection, which is why early antitoxin administration is critical (Nelson 21e, Harrison 21e Ch 142). ## Why each distractor is wrong - **Option 2 (Streptococcus pyogenes)**: While *S. pyogenes* causes exudative pharyngitis, it does not produce the characteristic adherent grey-white pseudomembrane. The hyaluronic acid capsule is an immune evasion mechanism, not the pathophysiology of the membrane itself. Streptococcal exudate is non-adherent and does not bleed when scraped. - **Option 3 (Epstein-Barr virus)**: EBV causes infectious mononucleosis with exudative pharyngitis and atypical lymphocytosis, but the exudate is non-adherent and does not extend as a fibrinous pseudomembrane. The clinical presentation lacks the characteristic grey-white adherent membrane of diphtheria. - **Option 4 (Candida albicans)**: Oral candidiasis presents with white patches (thrush) that are easily wiped off, not an adherent pseudomembrane. Candida does not cause systemic toxin-mediated disease or the systemic complications seen in diphtheria. **High-Yield:** Diphtheria toxin → EF-2 ADP-ribosylation → protein synthesis blockade → tissue necrosis → adherent pseudomembrane + systemic toxin effects (myocarditis, neuropathy). Early antitoxin is life-saving because it only neutralizes unbound toxin. [cite: Nelson 21e; Harrison 21e Ch 142]