A 4-year-old boy from rural Maharashtra presents with his mother to the outpatient clinic with multiple fluid-filled blisters on his left forearm and shin, which appeared 3 days ago. The lesions are painless, non-pruritic, and arranged in clusters. On examination, the blisters are flaccid, rupture easily, and leave a honey-crusted erosion. There is no fever or systemic involvement. A Gram stain of the blister fluid shows Gram-positive cocci in clusters. What is the most likely diagnosis?

Explanation

## Diagnosis: Bullous Impetigo ### Clinical Presentation The patient presents with the classic features of **bullous impetigo**: - **Flaccid blisters** that rupture easily (due to subcorneal location of the blister) - **Honey-crusted erosions** (pathognomonic finding) - **Non-pruritic, painless lesions** - **Clustered distribution** on exposed areas (forearm, shin) - **No systemic symptoms** (no fever) ### Microbiology & Pathogenesis **Key Point:** Bullous impetigo is caused by *Staphylococcus aureus* (classically), which produces **exfoliative toxins** (especially ETA and ETB). These toxins cleave **desmoglein-1** in the granular layer, causing subcorneal acantholysis and blister formation. **High-Yield:** Non-bullous impetigo (~70% of cases) is caused by *S. pyogenes* (Group A Streptococcus) or *S. aureus*, and presents with **honey-crusted erosions WITHOUT preceding blisters**. Bullous impetigo (~30% of cases) is almost always *S. aureus*. ### Gram Stain Finding Gram-positive cocci in **clusters** (grape-like arrangement) is characteristic of *Staphylococcus*. *Streptococcus pyogenes* would appear as cocci in **chains**. ### Differential Features | Feature | Bullous Impetigo | Non-bullous Impetigo | Pemphigus Vulgaris | Varicella | |---------|------------------|----------------------|--------------------|-----------| | **Blister type** | Flaccid, subcorneal | Erosion only | Flaccid, intraepidermal | Vesicles (tense) | | **Causative agent** | *S. aureus* | *S. pyogenes* or *S. aureus* | Autoimmune (anti-desmoglein-3) | VZV | | **Crust color** | Honey-golden | Honey-golden | No crust (erosion) | Hemorrhagic | | **Gram stain** | Gram-positive cocci in clusters | Gram-positive cocci in chains | Not applicable | Not applicable | | **Nikolsky sign** | Negative | Negative | **Positive** | Negative | | **Age group** | Children (2–6 years) | Any age | Adults (>40 years) | Any age | **Clinical Pearl:** The **flaccid nature** of the blister in bullous impetigo (due to subcorneal location) contrasts with the **tense blisters** of pemphigus (intraepidermal) and the **vesicles** of varicella (viral). ### Management - **Topical antibiotics:** Mupirocin 2% TID for 7–10 days (first-line for localized disease) - **Systemic antibiotics:** If extensive or non-responsive — amoxicillin-clavulanate or cephalexin (covers *S. aureus*) - **Hygiene:** Regular cleaning with soap and water **Warning:** Do NOT use topical corticosteroids alone — they worsen bacterial infections by impairing local immunity.