A 3-year-old unvaccinated child from a low-income urban settlement in Delhi presents with fever, sore throat, and a grayish-white pseudomembrane covering the tonsils and pharynx. Gram stain shows gram-positive bacilli. What is the most common site of diphtheria in this patient?

Explanation

## Sites of Diphtheria Infection **Key Point:** Pharyngeal diphtheria is the most common form, accounting for 80–90% of all diphtheria cases. The tonsils and pharynx are the primary sites of pseudomembrane formation. ### Anatomical Distribution of Diphtheria | Site | Frequency | Clinical Features | Severity | |------|-----------|---|---| | **Pharyngeal/Tonsillar** | 80–90% | Grayish-white pseudomembrane, sore throat, dysphagia | Most severe; high risk of airway obstruction | | **Laryngeal/Tracheal** | 5–15% | Croup-like symptoms, stridor, respiratory distress | Severe; risk of asphyxia | | **Cutaneous** | 1–5% | Ulcers, erosions, minimal systemic toxicity | Mild; rarely fatal | | **Nasal** | <1% | Serosanguineous discharge, crusting | Mild; often chronic | ### Why Pharyngeal Diphtheria Is Most Common 1. **Respiratory transmission** — *Corynebacterium diphtheriae* is spread via respiratory droplets; primary infection occurs in the upper respiratory tract 2. **Rich mucosal surface** — the pharynx and tonsils provide optimal conditions for bacterial adherence and toxin production 3. **Pseudomembrane formation** — the characteristic grayish-white pseudomembrane (composed of fibrin, bacteria, and necrotic epithelial cells) typically forms on the tonsils and extends to the pharynx 4. **Systemic toxicity** — pharyngeal diphtheria produces the highest concentration of diphtheria toxin, leading to severe myocarditis and neuropathy **High-Yield:** The pseudomembrane in diphtheria is **adherent** and **bleeds if forcibly removed**, distinguishing it from the exudate in streptococcal pharyngitis, which is non-adherent and easily wiped away. **Clinical Pearl:** Laryngeal diphtheria (croup) is the second most common form and represents a medical emergency due to risk of airway obstruction and asphyxia. Cutaneous diphtheria, though rare in developed countries, is more common in tropical regions and carries minimal systemic toxicity. **Warning:** Do not confuse diphtheria sites with diphtheria complications. Myocarditis and neuropathy (cranial nerve palsies, peripheral neuropathy) are complications of systemic toxin absorption, not sites of infection. **Mnemonic:** **PHON** = **PH**aryngeal (most common) → **O**ther sites (laryngeal, cutaneous, nasal) [cite:Park 26e Ch 12]