A 28-year-old woman presents to the emergency department with fever (38.5°C), severe oral pain, and difficulty swallowing for 3 days. She reports a maculopapular rash that started on her trunk 2 days ago and has now spread to her face, neck, and proximal limbs. On examination, she has extensive mucosal erosions involving the lips, buccal mucosa, and pharynx. Her skin shows targetoid lesions with central blistering and surrounding erythema. She was started on amoxicillin-clavulanate for a respiratory tract infection 5 days ago. Laboratory investigations show WBC 8,500/μL, hemoglobin 12.5 g/dL, and platelet count 180,000/μL. Skin biopsy shows full-thickness epidermal necrosis with minimal dermal inflammation. What is the most likely diagnosis?

## Diagnosis: Stevens-Johnson Syndrome (SJS) ### Clinical Presentation Analysis **Key Point:** SJS is characterized by the acute onset of fever, mucosal involvement, and a targetoid rash with <10% body surface area (BSA) involvement of blistering and detachment. This patient presents with: - **Mucosal involvement:** Extensive erosions of lips, buccal mucosa, and pharynx (hallmark of SJS/TEN) - **Systemic symptoms:** Fever, severe oral pain, dysphagia - **Skin findings:** Targetoid lesions with central blistering on trunk, face, neck, and proximal limbs - **Drug exposure:** Amoxicillin-clavulanate initiated 5 days prior (common culprit) - **Histopathology:** Full-thickness epidermal necrosis with minimal dermal inflammation (pathognomonic) ### SJS vs TEN Classification | Feature | SJS | TEN | SJS/TEN Overlap | |---------|-----|-----|------------------| | **BSA involvement** | <10% | >30% | 10–30% | | **Mucosal involvement** | Often present | Usually present | Present | | **Epidermal necrosis** | Full-thickness | Full-thickness | Full-thickness | | **Mortality** | 1–5% | 25–50% | 10–15% | | **Histology** | Minimal dermal inflammation | Minimal dermal inflammation | Minimal dermal inflammation | **High-Yield:** The extent of BSA involvement is the PRIMARY criterion for distinguishing SJS from TEN. This patient has rash involvement limited to trunk, face, neck, and proximal limbs—estimated <10% BSA—consistent with SJS. ### Pathophysiology **Clinical Pearl:** SJS/TEN is a cytotoxic T-lymphocyte (CTL)-mediated reaction. The drug (or its metabolite) acts as a hapten, binding to HLA molecules and presenting to CTLs. This triggers massive keratinocyte apoptosis, leading to full-thickness epidermal necrosis with relatively spared dermis. **Mnemonic: DRESS vs SJS** — **DRESS = Delayed Reaction with Eosinophilia and Systemic Symptoms** - DRESS: atypical lymphocytes, eosinophilia, lymphadenopathy, hepatosplenomegaly, skin involvement develops slowly (2–8 weeks) - SJS: acute onset (days), minimal dermal inflammation, no eosinophilia ### Management Principles 1. **Immediate:** Discontinue amoxicillin-clavulanate 2. **Supportive care:** IV fluids, electrolyte monitoring, pain control, eye care (risk of corneal ulceration) 3. **Systemic corticosteroids:** Controversial but often used in early SJS (within 48 hours of onset) 4. **ICU monitoring:** For fluid/electrolyte balance and infection surveillance 5. **Ophthalmology consultation:** Mandatory (risk of symblepharon, corneal scarring) **Warning:** Do NOT use NSAIDs or antibiotics (except for documented infection) in SJS/TEN—they may worsen disease. [cite:Robbins 10e Ch 25]