You are seeing a 18-year-old female in A&E who has presented with abdominal pain. During the history, the patient informs you that the pain came on suddenly several hours ago and was located in the centre of his abdomen but has now moved to the right iliac fossa. The pain is constant and sharp. The patient is also complaining of nausea, constipation and vomiting. On examination you note a tenderness in the right iliac fossa, facial flushing and a low-grade fever. What is the most likely diagnosis?