A 4-year-old girl from rural India presents with purulent pustules on the shin. Gram stain shows gram-positive cocci in clusters; culture grows methicillin-sensitive *Staphylococcus aureus*. Which finding most reliably distinguishes ecthyma (ulcerative impetigo) from superficial impetigo?

Explanation

## Ecthyma vs. Superficial Impetigo: The Depth Distinction ### Comparative Pathology | Feature | Superficial Impetigo | Ecthyma (Ulcerative Impetigo) | |---------|---------------------|------------------------------| | **Depth of invasion** | Stratum corneum + epidermis | Full epidermis → dermis | | **Primary lesion** | Pustule with honey crust | Vesicle → pustule → ulcer | | **Ulcer characteristics** | Absent | Punched-out, indurated, with eschar | | **Scarring** | Minimal (post-inflammatory marks) | Permanent (atrophic or hypertrophic) | | **Causative organism** | *S. aureus*, *S. pyogenes* | *S. aureus* (90%), *S. pyogenes* (10%) | | **Predisposing factors** | Poor hygiene, minor trauma | Malnutrition, poor hygiene, significant trauma | | **Healing time** | 1–2 weeks | 2–6 weeks | | **Lymphadenopathy** | Regional (mild) | Regional (prominent) | **Key Point:** Ecthyma is defined by **dermal invasion** resulting in a characteristic **punched-out ulcer with indurated borders and central eschar** — this is the single most reliable discriminator from superficial impetigo. ### Pathophysiology of Ecthyma 1. **Inoculation** → minor skin trauma (insect bite, abrasion, scratch) 2. **Bacterial invasion** → penetrates dermis (unlike superficial impetigo) 3. **Necrosis** → tissue destruction → ulcer formation 4. **Eschar formation** → central black/brown crust (necrotic tissue) 5. **Healing** → scarring (dermal involvement) **High-Yield:** The **punched-out ulcer with eschar** is pathognomonic for ecthyma. Superficial impetigo never ulcerates. **Mnemonic:** **ECTHYMA** = **E**pidermis + **D**ermis invaded (depth is key) ### Clinical Presentation **Ecthyma lesions:** - Start as painful vesicles on lower legs (shins, ankles) - Rapidly become pustular - Ulcerate within days - Surrounded by indurated, erythematous halo - Central eschar (black/brown necrotic tissue) - Tender and slow to heal - Leave permanent scars **Clinical Pearl:** Ecthyma is more common in malnourished children and those with poor hygiene in tropical/subtropical regions — very relevant in Indian dermatology practice. ### Why Depth Matters - **Superficial** → topical antibiotics may suffice - **Ecthyma** → requires systemic antibiotics + wound care - **Ecthyma** → higher risk of cellulitis, abscess formation [cite:Fitzpatrick's Dermatology 9e Ch 193]