A 72-year-old hypertensive man with chronic obstructive pulmonary disease and a 40-pack-year smoking history is admitted with acute-onset severe abdominal and back pain. A ruptured abdominal aortic aneurysm is confirmed on CT angiography. What is the most common underlying cause of abdominal aortic aneurysm in this patient population?

## Etiology of Abdominal Aortic Aneurysm **Key Point:** Atherosclerosis is the most common cause of abdominal aortic aneurysm, accounting for >90% of all AAA cases in developed countries. ### Pathogenesis of Atherosclerotic AAA 1. **Chronic inflammation and wall degradation**: Atherosclerotic plaques trigger chronic inflammatory infiltration (macrophages, T lymphocytes) in the aortic wall. 2. **Matrix metalloproteinase activation**: Inflammatory cells release MMPs (especially MMP-2 and MMP-9), which degrade elastin and collagen in the media and adventitia. 3. **Loss of structural integrity**: Degradation of the elastic laminae and smooth muscle loss lead to weakening and progressive dilation. 4. **Aneurysm formation**: The weakened wall expands under systemic pressure, creating the characteristic fusiform or saccular aneurysm. ### Risk Factors for Atherosclerotic AAA **Mnemonic: SHADES** — Smoking, Hypertension, Age (>60), Diabetes, Elevated cholesterol, Sex (male 5–10:1) **High-Yield:** The patient in this vignette has multiple risk factors: - Smoking (40-pack-year history) — **strongest modifiable risk factor** - Hypertension — present - Age >70 — present - Male sex — implied ### Comparison of AAA Etiologies | Cause | Frequency | Key Features | Typical Presentation | |-------|-----------|--------------|---------------------| | **Atherosclerosis** | >90% | Infrarenal, fusiform, males, age >60, smoking/HTN | Asymptomatic (screening) or rupture | | Syphilis (tertiary) | <1% | Ascending aorta, saccular, aortitis | Aortic regurgitation, aneurysm | | Marfan syndrome | <1% | Ascending aorta, cystic medial necrosis, young | Aortic dissection/rupture, family Hx | | Takayasu arteritis | <1% | Thoracic aorta, large-vessel vasculitis, young women | Claudication, reduced pulses | | Infectious (mycotic) | <1% | Any location, septic emboli | Fever, positive blood cultures | | Ehlers–Danlos syndrome | <1% | Thoracic/visceral, connective tissue disorder | Spontaneous rupture, young age | ### Why Atherosclerosis Dominates **Clinical Pearl:** Atherosclerotic AAA is almost exclusively **infrarenal** (95% of cases), occurring below the renal arteries. This is because: - The infrarenal aorta has fewer vasa vasorum (blood vessels in the aortic wall), making it more susceptible to atherosclerotic damage. - The infrarenal segment experiences higher hemodynamic stress. - Atherosclerotic disease is more prevalent in the distal aorta. **Warning:** Do not confuse atherosclerotic AAA with syphilitic aortitis: - Syphilitic aneurysms are **ascending aortic** and associated with aortic regurgitation (diastolic murmur, wide pulse pressure). - Syphilitic disease is now rare due to penicillin availability. [cite:Harrison 21e Ch 243; Robbins 10e Ch 10]