A 34-year-old woman presents to her general practitioner complaining of a rash. Over the past 2 weeks she has developed multiple tender red swellings on her shins and forearms. The older swellings are darker in colour and seem to be healing from the centre. She feels generally unwell and tired and also has pains in her wrists and ankles.

A 34-year-old woman presents to her general practitioner complaining of a rash. Over the past 2 weeks she has developed multiple tender red swellings on her shins and forearms. The older swellings are darker in colour and seem to be healing from the centre. She feels generally unwell and tired and also has pains in her wrists and ankles. She has not had a recent sore throat. Over the past 2 years she has had recurrent aphthous ulcers in her mouth. She has had no genital ulceration but she has been troubled by intermittent abdominal pain and diarrhoea. She works as a waitress and is unmarried. She smokes about 15 cigarettes per day and drinks alcohol only occasionally. She has had no other previous medical illnesses and there is no relevant family history that she can recall. She is thin but looks well. There are no aphthous ulcers to see at the time of the examination. Her joints are not inflamed and the range of movement is not restricted or painful. Examining the skin there are multiple tender lesions on the shins and forearms. The lesions are raised and vary from 1 to 3 cm in diameter. The fresher lesions are red and the older ones look like bruises. Physical examination is otherwise normal. Normal Haemoglobin 13.5 g/dL 11.7-15.7 g/dL White cell count 15.4 X 109/L 3.5-11.0 X 109/L Platelets 198 X 109/L 150-440 X 109/L Erythrocyte sedimentation rate (ESR) 98 mm/h <10 mm/h Sodium 138 mmol/L 135-145 mmol/L Potassium 4.3 mmol/L 3.5-5.0 mmol/L Urea 5.4 mmol/L 2.5-6.7 mmo