Cellulitis, abscess and necrotising soft-tissue infection: Viva

Branch 1: Classifying the infection and the purulent decision

The candidate should recognise that this child has both a purulent and a non-purulent element: a fluctuant abscess in the mid-forearm surrounded by a rim of cellulitis. The first decision, following the 2014 Infectious Diseases Society of America framework, is to separate purulent from non-purulent infection, because the therapy differs. The abscess is treated primarily by incision and drainage, while the surrounding cellulitis is treated with antibiotics directed at streptococci and staphylococci. [1]

The candidate should confirm the child is cardiovascularly stable and not toxic, which places her in the non-necrotising category for now, but should state explicitly the features that would trigger concern for necrotising infection: severe pain out of proportion to the visible erythema, systemic toxicity, tense woody induration, haemorrhagic bullae, crepitus, or rapid spread. The absence of these features in this child supports a purulent abscess with surrounding cellulitis rather than a surgical emergency. [3]

Branch 2: Managing the abscess and choosing antibiotics

The candidate should describe incision and drainage as the definitive therapy for the abscess, performed under adequate analgesia with an incision over the point of maximal fluctuance, loculation broken, pus expressed, and a sample sent for culture. The candidate should know that a simple, well-drained abscess in a well child often needs no antibiotic at all, but that this child warrants adjunctive antibiotics because of the surrounding cellulitis and the fever. [2]

The examiner will probe the antibiotic choice. Because this child has surrounding cellulitis and a fever, an oral agent such as cephalexin is appropriate first-line, with clindamycin or trimethoprim-sulfamethoxazole added or substituted if community-associated MRSA is prevalent locally. The candidate should justify the choice by local MRSA epidemiology and should arrange a review at 48 hours, because failure to improve would signal the wrong organism, an inadequately drained abscess, or progression to necrotising disease. [1]

Branch 3: Recognising progression to the surgical emergency

The examiner may pivot: the child returns at 24 hours with worsening pain, a spreading red margin, and new toxicity. The candidate must now recognise the red flags of necrotising infection and abandon the reassurance of a simple abscess. Severe disproportionate pain with systemic toxicity and rapid spread mandates immediate resuscitation, broad-spectrum intravenous antibiotics covering group A Streptococcus and MRSA, and urgent surgical review. [3]

The candidate should state that the LRINEC score and computed tomography may support the diagnosis but must never delay surgery when the clinical picture is convincing, and that urgent surgical debridement is the definitive therapy. A clear explanation that antibiotics and imaging are adjuncts to the knife, never substitutes for it, demonstrates the understanding that examiners are testing. [3]