## Diphtheria Pseudomembrane Pathology **Key Point:** The pseudomembrane of diphtheria is a pathognomonic finding consisting of fibrin, necrotic epithelial cells, and *Corynebacterium diphtheriae* embedded in a mucopolysaccharide matrix. ### Histological Composition The characteristic pseudomembrane contains: 1. **Fibrin** — forms the structural scaffold 2. **Necrotic epithelial cells** — from local tissue damage by diphtheria toxin 3. **Bacterial colonies** — *C. diphtheriae* organisms embedded throughout 4. **Mucopolysaccharide matrix** — provides the grayish-white, tough, adherent appearance 5. **Minimal purulent exudate** — unlike bacterial pharyngitis ### Why It's Called a "Pseudomembrane" | Feature | Pseudomembrane (Diphtheria) | True Membrane (e.g., Strep) | |---------|------------------------------|---------------------------| | **Composition** | Fibrin + necrotic epithelium + bacteria | Purulent exudate + fibrin | | **Adherence** | Firmly adherent, bleeds if removed | Loosely adherent | | **Color** | Grayish-white, tough | Yellow-white, softer | | **Underlying tissue** | Hemorrhagic, necrotic | Erythematous, inflamed | | **Toxin involvement** | Yes (diphtheria toxin) | No | **High-Yield:** The pseudomembrane is **firmly adherent** — attempting to remove it causes bleeding. This distinguishes it from the loosely adherent membrane of streptococcal pharyngitis. **Clinical Pearl:** The pseudomembrane typically begins on the tonsils and may extend to the pharynx, larynx, and trachea, potentially causing airway obstruction. The "bull neck" appearance results from cervical lymphadenopathy and soft tissue edema. **Mnemonic:** **FIBRIN** — Fibrin, Inflammatory necrosis, Bacteria embedded, In mucopolysaccharide, Necrotic epithelium [cite:Robbins 10e Ch 8]
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