A 3-year-old child presented to the emergency department with a 4-day history of fevers and refusal to weight bear. She had experienced a coryzal illness 7 days prior to the presentation, but was otherwise a normally developed, fully vaccinated child with no history of recurrent infections, travel history or sick contacts.On examination, she was febrile to 40 C, with otherwise appropriate parameters for her age. The right ankle joint was swollen and warm with restricted passive and active movement. The remainder of her clinical examination was unremarkable. The serum C-reactive protein was elevated at 74 g/L (normal = <8), with normal peripheral white cell counts and electrolytes. A blood culture was also taken. Right ankle radiograph showed no bony abnormality, and ultrasound demonstrated a moderate tibiotalar effusion.The patient was admitted under the orthopaedic team for immediate arthrotomy. Swabs were taken for Gram stain, culture and sensitivity, and she was commenced on empirical intravenous flucloxacillin at a dose of 200 mg/kg/day. Synovial fluid microscopy revealed a white cell count of 5000 ×10 6 /L with a polymorphonuclear proportion of 50%. The sample was heavily blood-stained with the presence of a fibrin clot.By the following day, the patient was afebrile and clinically improving. After 24 h of incubation, the peripheral blood culture Conflict of interest: None declared.A child with meningococcal arthritis C O'Rourke et al. Fig. 2Neisseria meningitidis isolated from the patient's blood culture, Gram stained and visualised with microscopy. The organisms can be seen forming diplococci that can be difficult to distinguish from coccobacilli or rods.

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