Severe Abdominal Pain Radiating to Back

A fit 72-year-old woman presented with a 12 h history of sudden onset of severe abdominal pain radiating to her back. With the onset of the pain she collapsed, and on recovering could not stand without feeling faint. On presentation the following day she had developed bruising of the abdominal wall.

Bruising of the abdominal wall

What is the most likely diagnosis ?