MSRA-0186

A 55-year-old woman is brought into the Emergency Department with a sudden onset of severe back pain lasting 30 minutes. The pain is constant and not exacerbated by coughing or sneezing.
On examination, the patient is in shock, with a palpable 7 cm mass deep in the epigastrium above the umbilicus. Her past medical history includes a 5 cm abdominal aortic aneurysm diagnosed three years ago at the time of appendectomy. The patient is a non-smoker and drinks one glass of wine a week.
What is the most likely diagnosis?