## Impetigo: Depth of Infection **Key Point:** Impetigo is a *superficial* pyoderma confined to the **epidermis** — specifically the stratum corneum and superficial stratum granulosum. It does NOT extend into the dermis; this distinguishes it from deeper pyodermas like erysipelas and cellulitis. ### Correct Statements Explained | Feature | Details | |---------|----------| | **Non-bullous form** | ~70% of cases; caused by *S. aureus* (increasingly MRSA) and *Streptococcus pyogenes* | | **Bullous form** | Caused by *S. aureus* strains producing exfoliative toxins (exfoliatin A/B); toxins cleave desmoglein-1 in the granular layer | | **Diagnosis** | Gram stain and culture of purulent exudate are gold standard; PCR can identify MRSA | ### Why Option 4 Is Wrong **High-Yield:** Impetigo is **intraepidermal** — it remains within the epidermis and does NOT breach the dermal-epidermal junction. The bullae in bullous impetigo form at the level of the granular layer due to toxin-induced acantholysis. Involvement of the dermis would indicate a deeper infection (erysipelas, cellulitis, or abscess formation). **Clinical Pearl:** This distinction is clinically important: impetigo typically lacks systemic symptoms (fever, lymphadenopathy are uncommon), whereas dermal involvement (erysipelas) presents with fever, chills, and regional lymphadenopathy. [cite:Robbins 10e Ch 25]
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