A 28-year-old woman develops a blistering eruption 2 weeks after starting trimethoprim-sulfamethoxazole for a urinary tract infection. Examination reveals confluent erythematous patches with central blistering on the palms, soles, and genitalia, along with oral mucosal erosions. Which single feature best distinguishes Stevens-Johnson Syndrome (SJS) from Toxic Epidermal Necrolysis (TEN) in this patient?

Explanation

## Distinguishing SJS from TEN: The Critical Threshold ### Key Discriminator: Body Surface Area (BSA) Involvement **Key Point:** The defining distinction between SJS and TEN is the extent of epidermal detachment, measured as percentage of total body surface area (BSA): - **SJS:** ≤10% BSA involvement - **SJS/TEN overlap:** 10–30% BSA involvement - **TEN:** >30% BSA involvement This is the single most important criterion used in clinical classification and prognostication. ### Shared Clinical Features (NOT discriminators) | Feature | SJS | TEN | |---------|-----|-----| | Oral mucosal erosions | Present (>90%) | Present (>90%) | | Ocular involvement | Common | Common | | Genital involvement | Common | Common | | Palmar/plantar involvement | Frequent | Frequent | | Prodrome (fever, malaise) | Yes | Yes | | Drug association | ~90% | ~90% | | Latency (7–21 days) | Typical | Typical | **Clinical Pearl:** All mucosal and anatomical site involvement patterns are shared between SJS and TEN. The *extent* of skin detachment is what separates them. ### Why BSA Matters 1. **Prognostication:** TEN (>30% BSA) carries mortality of 25–35%; SJS (<10% BSA) has mortality of 1–5%. 2. **Management intensity:** TEN typically requires ICU-level care; SJS may be managed on general wards. 3. **Systemic complications:** Organ involvement (renal, hepatic, pulmonary) correlates with BSA extent. **High-Yield:** Memorize the BSA cutoffs—they define the entire classification and are tested directly in NEET PG. ### Mnemonic: "SJS-TEN Triangle" **S**mall (SJS ≤10%) → **M**edium (Overlap 10–30%) → **T**otal (TEN >30%)