1) Most likely diagnosis: and why: Fusiform infrarenal abdominal aortic aneurysm (AAA) with mural thrombus. The axial CT shows focal, concentric dilation of the infrarenal aorta with an internal central contrast-filled lumen and eccentric peripheral low-attenuation material consistent with chronic mural thrombus, and curvilinear aortic wall calcification—classic features of a chronic fusiform AAA. No focal contrast extravasation or large retroperitoneal hematoma is seen on this single image to suggest active rupture. 2) Next best diagnostic step: Obtain a dedicated contrast-enhanced CT angiography (CTA) of the chest, abdomen and pelvis with arterial phase acquisition and multiplanar/3D reformats to measure maximum aneurysm diameter, neck length/angulation, and iliac anatomy, and to look comprehensively for signs of leak/retroperitoneal hematoma. If CTA contraindicated (severe renal dysfunction), duplex ultrasound (for size surveillance) or noncontrast CT plus limited imaging as clinically indicated may be used. 3) Key differential: or confirmatory test: Differentiate chronic degenerative AAA from pseudoaneurysm, aortic dissection with aneurysmal dilation, or an infected/mycotic aneurysm. CTA features and clinical correlation (fever, leukocytosis), blood cultures, and if infection suspected FDG-PET/CT or contrast-enhanced MRI can help confirm an infected aneurysm; for dissection/pseudoaneurysm CTA will generally provide definitive diagnosis. 4) Possible treatment: or management: Management depends on size, symptoms and anatomy: elective repair (endovascular aneurysm repair—EVAR—or open surgical repair) is recommended for most AAAs >5.5 cm in men, symptomatic aneurysms, or those with rapid growth. Smaller, asymptomatic aneurysms are managed with medical risk-factor control (BP control, smoking cessation, statin therapy) and interval imaging surveillance (ultrasound or CTA). If rupture is suspected clinically or on imaging, initiate resuscitation and urgent vascular intervention (typically emergent EVAR or open repair).