A 24-year-old woman presents with 12 hours of abdominal pain, nausea and anorexia. The pain began around the umbilicus and is now most severe in the right iliac fossa. She has no vaginal bleeding or discharge, urinary symptoms or previous abdominal surgery; a pregnancy test is negative. The tenderness is increased by passive extension of the right hip. Which one of the following is the most likely diagnosis?