Which feature best distinguishes diphtheria caused by toxigenic Corynebacterium diphtheriae from infection caused by non-toxigenic strains in a 7-year-old unvaccinated child presenting with pseudomembrane in the pharynx?

## Toxigenic vs Non-Toxigenic C. diphtheriae: Key Discriminator **Key Point:** The critical distinction between toxigenic and non-toxigenic strains lies in the production of diphtheria toxin, encoded by the β-phage lysogenic gene. This toxin causes systemic complications, not local infection. ### Comparison Table | Feature | Toxigenic Strain | Non-Toxigenic Strain | | --- | --- | --- | | **Pseudomembrane** | Present | Present | | **Local inflammation** | Present | Present | | **Myocarditis** | Yes (toxin-mediated) | No | | **Neuropathy** | Yes (toxin-mediated) | No | | **Gram stain/culture** | Gram-positive bacilli | Gram-positive bacilli | | **Cervical lymphadenopathy** | Present | Present | | **Systemic toxicity** | Severe | Absent/minimal | **High-Yield:** Non-toxigenic strains cause local pseudomembranous infection indistinguishable from toxigenic strains on clinical grounds alone. The pseudomembrane, cervical lymphadenopathy, and gram-positive bacilli on culture occur with both. Systemic toxin-mediated complications (myocarditis, cranial/peripheral neuropathy) are pathognomonic for toxigenic strains. **Clinical Pearl:** A child with pseudomembrane but NO cardiac arrhythmias or neurological signs over 2–3 weeks suggests non-toxigenic strain. Conversely, myocarditis (conduction blocks, heart failure) or neuropathy (palatal paralysis, ciliary paralysis) appearing 1–3 weeks into illness is virtually diagnostic of toxigenic diphtheria. **Mnemonic:** **TOXIN = Systemic Toxicity** — Toxigenic strains cause Toxin-mediated complications (myocarditis, neuropathy); non-toxigenic strains cause only local disease.