A 62-year-old patient has been delivered to the surgical department with complaints of sudden pain in the umbilical region irradiating to the back and groin, which was accompanied by a syncope. Objectively: the patient is in grave condition, umbilical region is tender on palpation, bowel sounds are diminished. AP drop is present. USI reveals: free fluid in the abdomen, thickening of the wall of the abdominal aorta. The most likely diagnosis is: