A 58-year-old hypertensive man presents with sudden-onset severe tearing chest pain radiating to the interscapular region. CT angiogram of the thorax shows a Stanford Type B aortic dissection with the intimal flap marked **A** in the descending aorta. Which of the following best describes the pathophysiological mechanism of the structure marked **A**?

Explanation

## Why Option 1 is right The intimal flap (structure **A**) in aortic dissection results from a tear in the aortic intima that permits blood to dissect into and within the media, creating two lumens: the true lumen (continuous with the original aortic channel) and the false lumen (the dissected space within the media). This is the defining pathophysiological feature of aortic dissection and is the mechanism that distinguishes dissection from other aortic pathology. The intimal flap itself is the boundary between these two lumens. (Harrison 21e, Ch 280) ## Why each distractor is wrong - **Option 2 (Thrombus causing stenosis)**: While thrombus can occur within a false lumen, the primary mechanism of dissection is not thrombosis but rather an intimal tear with medial dissection. Aortic stenosis is not the pathophysiology of dissection. - **Option 3 (Adventitial rupture with extravasation)**: This describes a contained aortic rupture, not a dissection. In dissection, the intima tears but the adventitia remains intact, containing the false lumen. Rupture involves breach of the adventitia. - **Option 4 (Cystic medial necrosis causing aneurysm)**: While cystic medial necrosis is a risk factor for dissection (seen in connective tissue disorders), it leads to aneurysm formation, not the acute intimal tear and medial dissection that characterizes aortic dissection. **High-Yield:** Stanford Type B dissection (descending aorta only, distal to left subclavian) is managed medically unless complicated; Type A (ascending aorta involvement) is a surgical emergency. [cite:Harrison 21e Ch 280]