Necrotising fasciitis — SCE Dermatology MCQ

Correct answer: D — Necrotising fasciitis

The correct answer is D, necrotising fasciitis. The combination of pain disproportionate to visible erythema, rapidly evolving dusky (violaceous) skin with bullae, high fever, tachycardia and a markedly raised CRP (310 mg/L) in a diabetic patient after minor trauma is the classic pattern of a rapidly progressive necrotising soft tissue infection, and this diagnosis must be actively excluded because delay in surgical debridement is the single biggest determinant of mortality. Diabetes is a recognised risk factor for necrotising fasciitis, and pain out of proportion to clinical signs together with systemic toxicity should prompt urgent surgical review rather than continued antibiotic-only management for presumed cellulitis. Imaging or scoring systems (such as LRINEC) should never delay surgical exploration when clinical suspicion is high. Why the other options are wrong: C. Uncomplicated cellulitis: Cellulitis causes tenderness proportionate to erythema and does not typically produce bullae, skin duskiness or this degree of systemic toxicity and CRP elevation. E. Erythema nodosum: This presents as tender erythematous nodules, usually on the shins, without bullae, fever of this severity or progressive skin necrosis. B. Superficial thrombophlebitis: This causes a tender palpable cord along a superficial vein with localised erythema, not diffuse dusky skin, bullae or septic physiology. A. Allergic contact dermatitis: This produces pruritic eczematous change at the site of allergen contact, not severe pain, bullae, fever or a markedly raised CRP. Key point: Pain disproportionate to visible skin changes, plus bullae, duskiness and systemic toxicity, mandates urgent exclusion of necrotising fasciitis with emergency surgical review, not reassurance that it looks like simple cellulitis.

Reference: Primary Care Dermatology Society (PCDS), Cellulitis, erysipelas and necrotising fasciitis clinical guidance, https://www.pcds.org.uk/clinical-guidance/cellulitis-and-erysipelas