You see Masie who is 8 years old in a busy afternoon surgery with her mum Clara. Clara is concerned that her daughter has come out in a non-itchy rash all over her lower legs over the course of the day. She has been complaining of intermittent headaches over the last one week which appear to have settled and did have some vague abdominal pain 2 days ago. Masie denies any photophobia, neck stiffness, vomiting or fevers. She is passing urine and opening her bowels as normal. She has no joint pain and has been going to school as normal.
On examination she has a symmetrical purpuric rash which is also present on her buttocks. Abdominal and neurological examinations and observations are normal.
What is the most likely diagnosis?