Erythema Multiforme: The Daily PANCE Blueprint
A 23-year-old man has a three-day eruption on his palms, dorsal hands, and forearms. The lesions are round and sharply marginated, each with a dusky center, a pale raised ring, and an outer red rim — "target" lesions. Ten days ago he had a cold sore on his lip. He has a few small erosions on the buccal mucosa but his eyes and genitals are uninvolved, less than 5% of his body surface is affected, there is no skin sloughing and Nikolsky sign is negative, and he is afebrile and feels well. He takes no medications. Which of the following is the most likely diagnosis?
A. Erythema multiforme
B. Stevens-Johnson syndrome
C. Urticaria
D. Bullous pemphigoid
E. Secondary syphilis
Answer and topic summary
The answer is A. Erythema multiforme
Round three-zone “target” lesions on the palms and extensor extremities, ten days after a herpes labialis outbreak, is erythema multiforme. The morphology is the diagnosis: a true target has three zones — a dusky or bullous center, a pale edematous ring, and an erythematous outer halo — and the distribution is acral and extensor, spreading centripetally, which is the reverse of most drug eruptions. Herpes simplex virus is the cause in the large majority of cases (Mycoplasma pneumoniae is the next most common, especially in children); this is a delayed cell-mediated response to viral DNA in the skin, which is why the rash arrives a week or more after the cold sore. EM minor like this is self-limited over two to four weeks and needs only symptomatic care — antihistamines, topical steroids, and analgesia — while recurrent EM is managed with suppressive antiviral therapy such as daily valacyclovir. Acyclovir started after the rash appears does not shorten the current episode. The distinction that matters clinically is Stevens-Johnson syndrome, and the discriminators are here in the stem: SJS is drug-triggered far more often than infectious, produces flat atypical targets on the trunk rather than raised acral ones, causes severe erosions across two or more mucosal surfaces with systemic illness and fever, has a positive Nikolsky sign with epidermal detachment, and is defined by body surface area — under 10% is SJS, over 30% is toxic epidermal necrolysis. This patient has none of that. The rest are morphology mismatches: urticaria is transient, migratory, and pruritic with individual wheals resolving within 24 hours; bullous pemphigoid is tense blisters on an older adult with a negative Nikolsky and a linear IgG and C3 band at the basement membrane; and secondary syphilis also loves the palms and soles but presents as a diffuse copper-colored papulosquamous rash with lymphadenopathy and constitutional symptoms, not three-zone targets.
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