So this is a transverse view of the abdominal aorta and you can see the anatomy pretty clearly here. You can see the liver, the IVC, and the aorta sitting on the vertebral bodies. There’s no aneurysm here, the aorta measures clearly under 3 cm, probably about 2, but if you look closely within the aortic lumen you can see some pathology.
So this is an aortic dissection, a descending or type II dissection, and you can see that flap waving around inside the lumen of the aorta.
Here’s a diagram showing the flap again with the vertebral body and the IVC, and you may occasionally see this. Ultrasound is actually pretty specific for dissection when you see it, but it’s not going to be super sensitive to rule it out, especially if you aren’t getting great visualization. Don’t rely on it to exclude dissection, but keep your eyes open, you may actually see a dissection and that can dramatically alter your care of the patient.