You are seeing a 20-year-old male in A&E who has presented with severe left sided hip pain. The patient reports that the pain appeared suddenly last night and that it has been getting worse to the point he could not sleep. The patient denies any trauma and has never experienced anything like this before. On examination the joint is swollen and red. It is also extremely tender on palpation and you struggle to complete any passive movements due to pain. What is the most important diagnosis to rule out in this patient?