Purpose <p>The paper aims to explore the complexities of the internal carotid artery (ICA) segmentation, particularly in the context of segmental agenesis, by presenting a unique case of ICA segmental agenesis without proximal regression. This case prompts a reevaluation of existing embryological classifications and suggests the need for an updated segmentation model.</p> Methods <p>A 63-year-old woman underwent brain Magnetic Resonance Imaging (MRI) and MR Angiography due to hypoacusis, revealing incidental findings of vascular anomalies, including a focal absence of contrast in the left ICA. Subsequent imaging techniques, including Computed Tomography Angiography (CTA) and Digital Subtraction Angiography (DSA), confirmed the segmental agenesis while also identifying associated vascular anomalies, such as a triplicated anterior communicating artery and a saccular aneurysm. The study employs a detailed analysis of embryological development and existing ICA classification systems to contextualize these findings.</p> Conclusions <p>The case highlights a previously unreported segmental agenesis of the ICA between the posterior communicating artery and the anterior choroidal artery, suggesting the existence of a new segment. The findings indicate that collateralization through a hyperplastic anterior choroidal artery and a complex anterior communicating system may offer compensation. This case advocates for an update in the ICA segmentation framework to accommodate emerging embryological insights and improve understanding of vascular anomalies.</p>