A 4-year-old boy from rural Uttar Pradesh presents with a 5-day history of sore throat, low-grade fever, and difficulty swallowing. On examination, he has a grayish-white pseudomembrane covering both tonsils and extending into the pharynx. The membrane is adherent and bleeds when attempts are made to remove it. His neck is swollen with prominent lymphadenopathy. He has not received the DPT vaccine. A throat swab is sent for culture. Which organism is most likely responsible, and what is the most appropriate initial management?

Explanation

## Diphtheria: Clinical Recognition and Emergency Management ### Clinical Presentation of Diphtheria **Key Point:** Diphtheria is a life-threatening toxin-mediated disease caused by *Corynebacterium diphtheriae*. The hallmark is a thick, grayish-white, adherent pseudomembrane that bleeds when attempts are made to remove it — this distinguishes it from the exudate of streptococcal pharyngitis, which is easily wiped off. ### Characteristic Features | Feature | Diphtheria | Strep Pharyngitis | Candidiasis | |---------|-----------|-------------------|-------------| | **Membrane** | Grayish-white, adherent, bleeds | Yellow-white, easily wiped off | White patches, easily wiped off | | **Onset** | Insidious (2–7 days) | Acute (1–3 days) | Gradual, often in immunocompromised | | **Fever** | Low-grade | High-grade (>38.5°C) | Minimal or absent | | **Systemic toxicity** | Marked (toxin-mediated) | Moderate | Minimal | | **Complications** | Myocarditis, neuritis, airway obstruction | Rheumatic fever, PSGN | Esophageal candidiasis | | **Organism** | *Corynebacterium diphtheriae* | *Streptococcus pyogenes* (GAS) | *Candida albicans* | ### Pathophysiology of Diphtheria Toxin 1. **Toxin production:** Only lysogenized strains (carrying the β-phage) produce diphtheria toxin. 2. **Mechanism:** Toxin inhibits elongation factor 2 (EF-2), blocking protein synthesis in host cells. 3. **Target organs:** - **Myocardium:** Myocarditis (arrhythmias, heart block, cardiogenic shock) - **Nervous system:** Cranial nerve palsies (soft palate, pharyngeal), peripheral neuropathy - **Airway:** Pseudomembrane can obstruct the airway, causing stridor and respiratory failure. ### Why Immediate Antitoxin Is Critical **High-Yield:** Diphtheria antitoxin must be administered **immediately** upon clinical suspicion — **do not wait for culture confirmation**. This is a medical emergency. **Reasons:** 1. **Toxin is irreversible:** Once diphtheria toxin binds to EF-2, the damage is done. Antitoxin only neutralizes circulating toxin, not toxin already bound to cells. 2. **Time-sensitive:** Mortality increases dramatically if antitoxin is delayed beyond 48 hours of symptom onset. 3. **Culture takes 24–48 hours:** Waiting for confirmation risks fatal complications (myocarditis, airway obstruction, neuritis). **Clinical Pearl:** The diagnosis of diphtheria is **clinical**, not microbiological. Culture is confirmatory but should never delay treatment. ### Management Algorithm ```mermaid flowchart TD A[Suspected diphtheria:<br/>Grayish pseudomembrane<br/>Adherent, bleeds on removal]:::outcome A --> B[Immediate actions]:::action B --> C[Administer diphtheria antitoxin<br/>IV or IM]:::urgent B --> D[Send throat swab for culture]:::action B --> E[Isolate patient<br/>Respiratory precautions]:::action B --> F[Supportive care:<br/>Airway monitoring<br/>Cardiac monitoring<br/>Nutritional support]:::action C --> G[Antibiotics:<br/>Penicillin G IV or<br/>Erythromycin/Azithromycin]:::action D --> H[Culture confirms<br/>C. diphtheriae]:::outcome G --> I[Monitor for complications:<br/>Myocarditis, neuritis,<br/>airway obstruction]:::decision ``` ### Antitoxin Administration **Key Point:** Diphtheria antitoxin is a horse serum product and carries a risk of serum sickness (5–10% of recipients). However, the risk of death from untreated diphtheria (>10%) far outweighs the risk of antitoxin. - **Dose:** 20,000–100,000 units IV (or IM if IV not feasible), depending on severity and duration. - **Timing:** As soon as clinical diagnosis is made. - **Skin test:** Perform intradermal test for hypersensitivity before IV administration (though this should not delay treatment in emergencies). ### Antibiotic Therapy **After antitoxin administration:** - **Penicillin G:** 1.2 million units IV every 4–6 hours for 7–10 days (preferred). - **Alternative:** Erythromycin 500 mg IV every 6 hours or azithromycin 500 mg daily. - **Eradicates organism** and prevents further toxin production. ### Why Other Options Are Wrong **Streptococcal pharyngitis** presents with high fever, easily wiped exudate, and rapid onset — not the insidious presentation with an adherent, bleeding pseudomembrane seen here. **Candidiasis** causes white patches (not a true pseudomembrane), minimal fever, and no systemic toxicity. **Waiting for culture** in suspected diphtheria is a fatal error — antitoxin must be given immediately.