A 6 year old, previously healthy boy called Oliver attended his general practitioner because of right shoulder pain that had developed over the course of one day. The GP does not know the family as they are new to the practice having moved from the USA a few years before. Oliver was a full term normal delivery, is fully immunised, and has no previous medical concerns. On examination Oliver looks generally well. He is not palpably febrile although the GP forgot to formally check his temperature. Oliver is reluctant to use his right arm but his other joints are normal. There is no rash.

The GP wonders whether Oliver has transient synovitis secondary to a viral infection, but he has seen this most frequently affecting the hip joint rather than the shoulder. He decides that the most likely diagnosis is a trivial injury or an awkward sleeping posture. He explains this to Oliver’s mother and outlines the differential diagnosis. The GP decides not to treat with a non-steroidal anti-inflammatory drug (NSAID) as he is concerned that it may mask any pain due to inflammatory joint disease. He arranges to review Oliver if there is no improvement over the next few days.

The following day Oliver becomes generally unwell with fever, nausea and vomiting. He also develops redness near the joint. His mother contacts the GP as advised and is seen a few hours later at the end of surgery. Oliver’s temperature is 38°C and he is even more reluctant to move his arm. There is an area of erythema measuring 1 × 2 cm in the right axilla.

The GP considers septic arthritis in his differential diagnosis but decides that the joint pain is not severe enough and that the most likely diagnosis is cellulitis. He prescribes oral flucloxacillin (125 …