Pinpointing the rare cause of a painful, swollen knee

Is it a septic spectre or something else?

Jenny is a 16-year-old brought in by her mother with pain and swelling in her right knee.

The pain began a day or so ago, and the swelling appeared yesterday. Jenny feels unwell, and her mother recorded a fever of 37.8°C this morning.

She denies any recent diarrhoea, abdominal pain or associated rash, although her acne has been flaring recently. There is no family history of inflammatory arthritis or connective tissue disorders.

Jenny’s mother is asked to leave the consult for a brief period, during which