This prospective observational study aims to investigate whether serum REST4 concentration can serve as a quantitative indicator correlated with abdominal aortic aneurysm (AAA) diameter and a predictive biomarker for AAA progression and rupture risk. Emerging evidence suggests that REST4, a splice variant of the RE-1 Silencing Transcription Factor (REST), is upregulated in vascular smooth muscle cells during AAA pathogenesis and may be released into circulation via exosomes. The study will enroll patients with asymptomatic AAA (diameter between 39-49 mm) and measure baseline serum REST4 levels, analyzing their association with morphological severity (maximum aortic diameter) and clinical outcomes (aneurysm growth rate, rupture and related mortality) through a 5-year follow-up period.