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?

Explanation

## Why Arterial embolism from a cardiac source is right The clinical presentation and imaging findings are pathognomonic for SMA embolism. The patient has atrial fibrillation (a major cardiac source of emboli), acute onset pain out of proportion to physical findings (classic for acute mesenteric ischemia), and critically, the occlusion is located 6 cm distal to the SMA origin with **patent first jejunal branches**. This pattern is the hallmark of arterial embolism: the embolus typically lodges 3–10 cm from the SMA origin, sparing the proximal branches. In contrast, thrombosis occurs at the SMA origin itself, causing more severe proximal ischemia. The presence of atrial fibrillation further supports an embolic source (Gray's Anatomy 42e Ch 60; Bailey & Love 28e). ## Why each distractor is wrong - **Arterial thrombosis superimposed on atherosclerosis**: Thrombosis occurs at the SMA origin (not 6 cm distal), causing occlusion of all branches including the first jejunal vessels. Patients typically have a prodrome of intestinal angina (postprandial pain, food avoidance, weight loss), not acute onset. The patent proximal branches rule out thrombosis. - **Non-occlusive mesenteric ischemia from septic shock**: NOMI presents with patent vessels on imaging; there is no occlusion here. The CT shows a discrete thrombus obstructing the vessel, not a low-flow state. NOMI occurs in shock states (sepsis, cardiogenic shock, severe dehydration), not in a patient with atrial fibrillation alone. - **Mesenteric venous thrombosis from Factor V Leiden mutation**: MVT typically presents in younger patients with hypercoagulable states and has a slower onset over days (not acute). The imaging shows arterial occlusion, not venous thrombosis. There is no mention of risk factors for thrombophilia in this elderly patient with atrial fibrillation. **High-Yield:** SMA embolism lodges 3–10 cm distal to origin, sparing proximal branches; SMA thrombosis occurs at origin, occluding all branches — this distinction is critical for diagnosis and management. [cite: Gray's Anatomy 42e Ch 60; Bailey & Love 28e]