A 32-year-old woman presents with extensive blistering, mucosal erosions, and fever 10 days after starting trimethoprim-sulfamethoxazole for a urinary tract infection. Skin biopsy shows full-thickness epidermal necrosis with subepidermal blister formation. On examination, 35% of her body surface area shows epidermal detachment. Which feature best distinguishes her condition from Stevens-Johnson Syndrome?

Explanation

## Clinical Diagnosis: TEN vs. SJS ### Case Analysis The patient has: - **35% BSA epidermal detachment** — this is the critical finding - Full-thickness epidermal necrosis on biopsy - Mucosal involvement (erosions) - Drug exposure (TMP-SMX) 10 days prior - Systemic toxicity (fever) The **35% BSA involvement** definitively places this patient in the **TEN category** (>30% BSA), not SJS (<10% BSA). ### Discriminating Feature: BSA Threshold **Key Point:** The extent of epidermal detachment as a percentage of total body surface area is the single best discriminator between SJS and TEN. ```mermaid flowchart TD A[Acute drug reaction with epidermal necrosis]:::outcome --> B{Percentage of BSA<br/>with epidermal detachment?}:::decision B -->|< 10%| C[Stevens-Johnson Syndrome]:::outcome B -->|10-30%| D[SJS/TEN Overlap Syndrome]:::outcome B -->|> 30%| E[Toxic Epidermal Necrolysis]:::outcome E --> F[Mortality risk 25-50%]:::urgent C --> G[Mortality risk < 5%]:::action ``` ### Why Other Features Do NOT Discriminate | Feature | SJS | TEN | Discriminates? | |---------|-----|-----|----------------| | Full-thickness epidermal necrosis | ✓ | ✓ | **No** | | Mucosal involvement | >90% | >90% | **No** | | Drug causation | ~80% | ~80% | **No** | | Fever/systemic toxicity | ✓ | ✓ | **No** | | **BSA detachment** | **<10%** | **>30%** | **Yes** | **High-Yield:** In this case, the 35% BSA involvement immediately identifies TEN, not SJS. This is the highest-yield discriminator for NEET PG exams. ### Clinical Pearl Histologic findings (full-thickness necrosis, subepidermal blister) are identical in both SJS and TEN. The clinical extent (BSA) is what separates them and determines prognosis and management intensity. [cite:Harrison 21e Ch 56]