A 35-year-old woman with a history of penicillin anaphylaxis is diagnosed with cutaneous diphtheria caused by Corynebacterium diphtheriae. What is the drug of choice for her treatment?

Explanation

## Diphtheria Treatment in Penicillin-Allergic Patients ### First-Line for Penicillin Allergy: Erythromycin **Key Point:** Erythromycin is the drug of choice for Corynebacterium diphtheriae infection in patients with documented penicillin allergy or anaphylaxis. **High-Yield:** Erythromycin is a macrolide antibiotic with excellent activity against C. diphtheriae and achieves good tissue penetration. It is bacteriostatic but effective in both acute diphtheria and carrier eradication. ### Dosing and Duration - **Acute diphtheria:** Erythromycin 500 mg IV/PO four times daily for 7 days - **Carrier eradication:** Erythromycin 500 mg PO four times daily for 7 days (or rifampicin 600 mg daily × 7 days as alternative) - **Cutaneous diphtheria:** Lower risk of systemic toxin; oral erythromycin is often sufficient ### Why Erythromycin in Penicillin Allergy? | Feature | Erythromycin | Cephalosporin | Vancomycin | Chloramphenicol | |---------|--------------|---------------|-----------|------------------| | **Cross-reactivity with PCN** | Minimal (~1–3%) | 1–3% (low risk) | None | None | | **Activity vs C. diphtheriae** | Excellent | Good | Excellent | Good | | **Carrier eradication** | Yes | No | No | No | | **First-line in PCN allergy** | **Yes** | Conditional | Reserved | Rarely used | | **Toxicity profile** | GI upset, QT prolongation | Generally safe | Nephrotoxicity, ototoxicity | Bone marrow suppression | **Clinical Pearl:** Although cephalosporins have low cross-reactivity with penicillins (~1–3%), they are NOT preferred in documented anaphylaxis. Erythromycin is safer and equally effective. **Warning:** Vancomycin and chloramphenicol are reserved for serious infections or when macrolides are contraindicated (e.g., severe liver disease, QT prolongation). They are NOT first-line alternatives in simple penicillin allergy. ### Cutaneous vs. Respiratory Diphtheria **Mnemonic:** **CUTANEOUS = Carrier eradication, Uncomplicated course, Topical care, Antibiotic choice is erythromycin, Neuropathy rare, Eradication easier, Often outpatient, Urgent antitoxin NOT needed, Systemic toxin low** - **Cutaneous diphtheria:** Lower systemic toxin risk; oral erythromycin sufficient; antitoxin not always needed - **Respiratory diphtheria:** High systemic toxin risk; IV antibiotics + antitoxin mandatory; ICU monitoring for myocarditis and airway obstruction **High-Yield:** In cutaneous diphtheria, erythromycin monotherapy (without antitoxin) is often adequate because toxin production is localized and systemic absorption is minimal. [cite:Park 26e Ch 8; Textbook of Microbiology (Ananthanarayan & Paniker) Ch 18]