CT Angiogram Abdominal Aorta

A 68-year-old man with atrial fibrillation presents to the emergency department with acute onset severe abdominal pain. On examination, his abdomen is soft with minimal tenderness, but his pain severity is markedly disproportionate to the clinical findings. CT angiography of the abdomen shows occlusion of the vessel marked **B** in the diagram, with the thrombus lodged 6 cm distal to its origin at the L1 vertebral level. The first jejunal branches are patent. Which of the following is the most likely etiology of this acute mesenteric ischemia?