A 6-year-old girl from Delhi presents with her father with multiple honey-coloured crusted lesions on her face, neck, and exposed parts of the arms for the past 10 days. The lesions started as small red papules and progressed to pustules before crusting over. There is mild regional lymphadenopathy. On examination, the crusts are thick and adherent. Culture of the exudate grows β-lactamase–producing *Staphylococcus aureus*. What is the most appropriate first-line antibiotic for this patient?

Explanation

## Management of Non-bullous Impetigo: First-Line Therapy ### Clinical Context **Key Point:** The clinical presentation—thick, adherent crusts; progression from papule → pustule → crust; and localized distribution—is classic for non-bullous impetigo. The β-lactamase–producing *S. aureus* isolate rules out penicillin-based monotherapy. **High-Yield:** For localized impetigo (< 5 lesions or limited body surface area), **topical antibiotics are first-line**. Mupirocin is the gold standard topical agent for impetigo. ### Why Mupirocin Is the Correct Choice 1. **Mechanism of action:** Mupirocin inhibits bacterial isoleucyl-tRNA synthetase, preventing protein synthesis. It is bactericidal against *S. aureus* and *S. pyogenes*. 2. **Efficacy:** Cure rates of 85–95% for non-bullous impetigo when applied topically. 3. **Guideline recommendation:** WHO, AAD, and Indian Academy of Dermatology recommend topical mupirocin as first-line for localized impetigo. 4. **Resistance:** Mupirocin resistance in *S. aureus* remains low (< 5% globally). 5. **Advantages:** Minimal systemic absorption, low risk of adverse effects, cost-effective, and prevents spread. ### Impetigo Management Algorithm ```mermaid flowchart TD A[Non-bullous Impetigo Diagnosis]:::outcome --> B{Extent of Disease?}:::decision B -->|Localized: < 5 lesions or < 2% BSA| C[Topical Mupirocin]:::action B -->|Extensive: > 5 lesions or > 2% BSA| D[Systemic Antibiotic]:::action B -->|Bullous or Systemic Signs| E[Systemic Antibiotic]:::action C --> F[Apply TID for 7–10 days]:::action D --> G{β-lactamase Producer?}:::decision E --> G G -->|Yes| H[Cephalosporin or Fluoroquinolone]:::action G -->|No| I[Amoxicillin or Penicillin V]:::action H --> J[Review at 48 hrs]:::decision I --> J J -->|Improved| K[Continue to cure]:::outcome J -->|No improvement| L[Switch to alternative]:::urgent ``` ### Why Other Options Are Incorrect | Option | Why It Is Wrong | |--------|----------------| | **Amoxicillin** | β-lactamase–producing *S. aureus* will hydrolyze amoxicillin, rendering it ineffective. This is a trap answer for students who forget the resistance mechanism. | | **Cefazolin** | While cefazolin is a valid systemic option for extensive impetigo, it is NOT first-line for localized disease. Topical therapy is preferred to minimize systemic exposure and cost. | | **Clindamycin** | Clindamycin is a reasonable alternative for MRSA impetigo, but it is NOT first-line. Mupirocin topical is preferred for localized disease. Clindamycin is reserved for systemic or extensive impetigo. | **Clinical Pearl:** The presence of regional lymphadenopathy (lymphangitis) does NOT mandate systemic therapy if the skin lesions are localized and the child is systemically well. Topical mupirocin with close follow-up is still appropriate. **Warning:** Do not confuse "β-lactamase–producing" with "MRSA." β-lactamase–producing *S. aureus* is typically methicillin-susceptible and will respond to cephalosporins or fluoroquinolones. True MRSA requires clindamycin, fluoroquinolones, or linezolid. ### Dosing and Duration - **Mupirocin ointment:** Apply a thin layer to affected areas **three times daily** for **7–10 days**. - **Cure rate:** 85–95% with appropriate topical therapy. - **Follow-up:** Review at 48 hours; if no improvement, switch to systemic antibiotic. [cite:Park 26e Ch 5; IAD Guidelines on Impetigo 2022]