James is an 85-year-old male who presents to the er complaining of a "strange and pulsating sensation in the middle of my abdomen, near my belly button. " he states it has been occurring for the past three days as well as deep pain in his lower back. auscultation of the abdomen revealed a significant bruit over the aorta and palpation revealed some tenderness and a pulsatile mass . what is your initial diagnosis?