USMLE Step 1 · Dermatology

Erythema multiforme vs SJS and TEN

How to tell Erythema multiforme and SJS and TEN apart when a board question makes them look alike.

Fastest separator

EM is target lesions with limited detachment; SJS is severe mucosal disease with under 10% BSA detachment; TEN is over 30% BSA detachment.

Side by side

Erythema multiformeSJS and TEN

Key clue

Erythema multiforme causes target lesions, often after HSV or Mycoplasma, with limited mucosal involvement and minimal epidermal detachment.

SJS and TEN cause severe mucocutaneous necrosis, fever, painful dusky lesions, positive Nikolsky sign, and epidermal detachment, often drug-triggered.

Mechanism

Immune reaction to infection or medication creates acral target lesions with limited necrosis.

Drug-triggered cytotoxic T-cell apoptosis of keratinocytes causes full-thickness epidermal necrosis; SJS is under 10% BSA, TEN over 30%.

Labs and tests

Classic three-zone target lesions on extremities support erythema multiforme.

Mucosal erosions, skin pain, Nikolsky sign, and BSA detachment support SJS/TEN.

The trap

Targetoid lesions can overlap, but skin pain, mucosal severity, Nikolsky, and BSA detachment separate SJS/TEN.

Watch it explained

60-second lessonErythema Multiforme vs SJS vs TEN: The 10 and 30 Percent Rule (USMLE Step 1)

Quiz yourselfSJS or TEN? 6-Second Dermatology Pop Quiz (USMLE Step 1)

Test it now

The repair move

For targetoid drug eruptions, quantify mucosal injury and body-surface detachment in three steps: (1) typical TARGET LESIONS (three-zone bullseye) with minimal mucosal involvement and no significant epidermal detachment equals erythema multiforme (most commonly triggered by HSV, also Mycoplasma; usually self-limited; treat trigger); (2) BSA detachment UNDER 10 percent plus severe mucosal involvement (at least 2 sites: oral, ocular, genital, anorectal) plus systemic toxicity equals Stevens-Johnson syndrome (drug-induced: sulfa drugs, anticonvulsants like lamotrigine and carbamazepine, allopurinol, NSAIDs; treat with immediate drug withdrawal, supportive care often in burn unit, ophthalmology consult for ocular involvement; controversial role for IVIG and cyclosporine); (3) BSA detachment OVER 30 percent equals toxic epidermal necrolysis (TEN) same drug list as SJS but more severe full-thickness epidermal necrosis with positive Nikolsky sign; SJS/TEN OVERLAP is 10 to 30 percent BSA detachment. SCORTEN score predicts mortality. Treat in a burn ICU with aggressive fluid resuscitation, infection prevention, and stop the offending drug immediately.

Try a free question

Core

A 28-year-old woman 2 weeks after starting sulfamethoxazole-trimethoprim for a UTI develops symmetric, well-demarcated target lesions on the palms, soles, and dorsal hands, with mild oral involvement of two small ulcers. There is no skin detachment. She is afebrile. Which diagnosis is most likely?

Show answer and explanation

Answer: A. Erythema multiforme. Erythema multiforme features symmetric target lesions, predominantly on extremities, with limited or no mucosal involvement and NO significant skin detachment; commonly triggered by HSV infection or drugs; treatment is supportive.

Why the other choices are tempting

B. Stevens-Johnson syndrome has severe mucosal involvement plus epidermal detachment <10% BSA.
C. Toxic epidermal necrolysis has detachment >30% BSA with high mortality.
D. Urticaria has migrating wheals without target morphology or mucosal involvement.
E. Pemphigus vulgaris has flaccid bullae with mucosal erosions and positive Nikolsky.

Flashcards

Tap a card to flip it.

What BSA cutoff defines TEN?

TEN involves more than 30% body-surface epidermal detachment.

What trigger often precedes erythema multiforme?

HSV infection often precedes erythema multiforme.

Quick answers

What is the fastest way to tell Erythema multiforme from SJS and TEN?