Erythema nodosum — SCE Dermatology MCQ

Correct answer: A — Erythema nodosum

The correct answer is A, erythema nodosum: tender, erythematous nodules confined to the anterior shins, occurring two weeks after a streptococcal sore throat and accompanied by ankle arthralgia, are the classic presentation of this septal panniculitis. Erythema nodosum is a delayed hypersensitivity reaction and streptococcal throat infection is one of the most common identifiable triggers, alongside sarcoidosis, inflammatory bowel disease and drugs. The lesions are typically bilateral, non-ulcerating, pretibial nodules that resolve without scarring, distinguishing them from other panniculitides, and concurrent arthralgia is a well recognised systemic feature. The absence of ulceration and livedo in the stem specifically excludes the vasculitic and vasculopathic differentials, reinforcing erythema nodosum as the diagnosis. Why the other options are wrong: B. Nodular vasculitis: this typically affects the calves rather than the shins, is associated with tuberculosis (erythema induratum) and characteristically ulcerates, which this patient does not have. E. Cellulitis: presents as a unilateral, spreading, warm area with systemic signs of infection (fever, malaise) rather than bilateral discrete nodules following a preceding sore throat. D. Pyoderma gangrenosum: begins as a pustule or nodule that rapidly progresses to a painful ulcer with a violaceous, undermined edge; the stem explicitly states there is no ulceration. C. Necrobiosis lipoidica: produces atrophic, shiny, yellow-brown plaques with telangiectasia, usually in patients with diabetes, not tender acute nodules following an infective trigger. Key point: bilateral tender pretibial nodules with arthralgia after a streptococcal sore throat, without ulceration or livedo, point to erythema nodosum as a septal panniculitis triggered by infection.

Reference: DermNet NZ, Erythema Nodosum, https://dermnetnz.org/topics/erythema-nodosum