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

Explanation

## Diphtheria Toxin Mechanism **Key Point:** Diphtheria toxin is an AB-type exotoxin produced by lysogenic strains of *Corynebacterium diphtheriae* (those infected with β-phage carrying the *tox* gene). ### Mechanism of Action 1. **Binding and Entry:** Toxin binds to heparin-binding EGF-like growth factor (HB-EGF) receptor on host cells 2. **Translocation:** A-chain enters cytoplasm and catalyzes ADP-ribosylation of elongation factor 2 (EF-2) 3. **Effect:** Inhibits protein synthesis → cell death ### Clinical Manifestations | Feature | Mechanism | |---------|----------| | Pseudomembrane (throat/nasopharynx) | Local tissue necrosis from toxin | | Myocarditis | Systemic toxin absorption → cardiac myocyte damage | | Neuropathy (cranial nerves, motor) | Demyelination from toxin effect on Schwann cells | | Respiratory obstruction | Membrane extension to larynx/trachea | **High-Yield:** Only *lysogenic* strains (carrying tox^+ phage) produce toxin. Non-lysogenic strains cause only localized infection without systemic toxicity. **Clinical Pearl:** The pseudomembrane is **not** pus but a fibrin-rich necrotic layer with bacteria, RBCs, and WBCs. It is firmly adherent and bleeds when forcibly removed. [cite:Park 26e Ch 34]