Which of the following is the defining pathological feature that distinguishes Stevens-Johnson Syndrome (SJS) from Toxic Epidermal Necrolysis (TEN)?

Explanation

## Distinguishing SJS from TEN **Key Point:** The ONLY reliable criterion to differentiate SJS from TEN is the extent of epidermal detachment as a percentage of body surface area (BSA). ### Classification by BSA Involvement | Condition | BSA with Epidermal Necrosis | Clinical Features | |-----------|------------------------------|-------------------| | SJS | < 10% | Mucosal involvement, target lesions, slower progression | | SJS/TEN overlap | 10–30% | Intermediate presentation | | TEN | > 30% | Extensive detachment, rapid progression, higher mortality | **High-Yield:** This morphological distinction is the gold standard used in all major dermatology textbooks and clinical practice. It is NOT based on histology alone, clinical severity, or mucosal involvement—those are associated features but not the defining criterion. **Clinical Pearl:** Both SJS and TEN show identical histopathology (full-thickness epidermal necrosis with subepidermal blister), so histology cannot differentiate them. The distinction is purely quantitative (extent of detachment). **Warning:** Common misconception—students often think mucosal involvement or target lesions define SJS. In fact, mucosal involvement occurs in both SJS and TEN; it is the BSA percentage that determines the diagnosis.