Katy was a 6-year-old girl who first presented to the local paediatric minor injuries centre with a history of a swollen, painful right knee. This was of “a couple of days” duration, and the family could not recall any traumatic events in the preceding days. She had a mild upper respiratory tract infection. There was no family history of arthritis, psoriasis or inflammatory bowel disease. The examining practitioner observed Katy walking into the consultation room slowly and with an antalgic gait. She was noted to be rather quiet but did not look toxic. She denied any joint pain when asked specifically. She was rather flushed, but had no rash, and her temperature was 37.5°C. She had a swollen right knee with an effusion. There was no bruising, but the joint was warm to touch and had a reduced range of movement. Remaining detailed systemic examination, including urinalysis, blood pressure and assessment of height and weight that were plotted on appropriate centile charts, was normal. Katy was referred that day to the orthopaedic team at the nearby district general hospital, with possible septic arthritis as the working diagnosis.

The examining practitioner was aware that acute septic arthritis is a medical emergency with the potential to cause rapid and irreversible joint damage, so must always be considered in any child with a new presentation of arthritis. Bacterial seeding to the joint via haematogenous transmission is the most frequent pathophysiological process, so he looked specifically for signs of systemic toxicity suggestive of bacterial sepsis.

The commonest cause of acute arthritis is reactive arthritis, and there was evidence of an upper respiratory tract infection. Inflammatory joint disease can be associated with other systemic illnesses such as connective tissue disorders and inflammatory bowel disease, signs of which were looked for on detailed multisystem examination, which …