A 6-year-old unvaccinated child from rural India presents with a sore throat, low-grade fever, and a thick, grayish-white pseudomembrane covering the tonsils and pharynx that bleeds when scraped. Blood culture is negative. What is the most likely causative organism?

Explanation

## Diagnosis: Diphtheria **Key Point:** The clinical presentation of a thick, grayish-white pseudomembrane (not true membrane) that is adherent and bleeds when scraped, combined with systemic symptoms in an unvaccinated child, is pathognomonic for diphtheria caused by *Corynebacterium diphtheriae*. **Pathophysiology:** - *C. diphtheriae* produces diphtheria toxin (via lysogenic β-phage) that inhibits protein synthesis by inactivating elongation factor 2 (EF-2) - The pseudomembrane is composed of fibrin, bacteria, and necrotic epithelial cells - Negative blood culture is typical—diphtheria is a localized infection with systemic toxin effects **Clinical Pearl:** The pseudomembrane is the hallmark feature—it is: - Thick and leathery - Adherent (does not wipe off easily) - Bleeds when forcibly removed - Grayish-white or yellowish in color - May extend to nasopharynx, larynx, or trachea **High-Yield:** Diagnosis is clinical; culture on Loeffler's medium or tellurite agar confirms it. Toxin production is confirmed by Elek test. Treatment must not be delayed for culture results.