A 52-year-old woman with a history of smoking and uncontrolled hypertension presents to the emergency department with the worst headache of her life, onset during Valsalva while straining at stool. Non-contrast CT head shows hyperdense blood in the basal cisterns in a characteristic star-pattern, marked as **A** in the diagram. Which of the following aneurysm locations is MOST likely responsible for this radiologic finding?

Explanation

## Why Anterior communicating artery (AComm) is right The star-pattern hyperdense blood in the basal cisterns (**A**) is the classic radiologic hallmark of aneurysmal subarachnoid hemorrhage (SAH). The anterior communicating artery (AComm) is the most common site of saccular (berry) aneurysm rupture, accounting for 35% of all aneurysmal SAH cases. AComm aneurysms rupture directly into the basal cisterns and interhemispheric fissure, producing the characteristic star-pattern distribution of blood. This patient's thunderclap headache, smoking history, and hypertension are classic risk factors for aneurysmal SAH. The radiologic anchor—the star-pattern in basal cisterns—directly correlates with AComm rupture in the majority of cases (Harrison 21e Ch 432; Greenberg 9e). ## Why each distractor is wrong - **Basilar artery tip**: While basilar artery aneurysms can cause SAH, they typically produce blood in the suprasellar cistern and perimesencephalic region rather than the classic basal cistern star-pattern. They account for <10% of aneurysmal SAH. - **Vertebral artery junction**: PICA-origin aneurysms and vertebral artery aneurysms produce SAH with blood predominantly in the posterior fossa and perimesencephalic cisterns, not the basal cistern star-pattern. - **Posterior inferior cerebellar artery (PICA)**: PICA aneurysms are rare (<5% of SAH) and typically present with posterior fossa hemorrhage and brainstem signs, not the characteristic basal cistern star-pattern seen with anterior circulation aneurysms. **High-Yield:** Star-pattern SAH in basal cisterns = AComm aneurysm (35%) or PCom aneurysm (30%); convexity SAH = amyloid angiopathy; perimesencephalic SAH = benign venous bleed. [cite: Harrison 21e Ch 432; Greenberg Handbook of Neurosurgery 9e]