A 52-year-old Indian woman with a history of smoking and hypertension presents to the emergency department with a sudden-onset "thunderclap" headache described as "the worst headache of my life" that reached maximum intensity within seconds. She has neck stiffness and photophobia. Non-contrast CT head shows the pattern marked **A** in the diagram—diffuse hyperdense blood in the basal cisterns consistent with a ruptured berry aneurysm. Which of the following is the MOST COMMON location of the aneurysm responsible for this presentation?

Explanation

## Why Anterior communicating artery (35%) is right The clinical anchor states that the MOST COMMON cause of non-traumatic SAH is rupture of a saccular (berry) aneurysm at branch points of the Circle of Willis. The AHA/ASA 2023 Guidelines explicitly establish that the anterior communicating artery (AComm) is the site of 35% of all aneurysmal SAH, making it the single most common location. The pattern marked **A**—diffuse blood in the basal cisterns—is the classic CT signature of aneurysmal SAH, and the question tests whether the student knows the epidemiology of aneurysm locations. This is a high-yield fact for NEET-PG exams and essential for clinical triage and neurosurgical planning. ## Why each distractor is wrong - **Posterior communicating artery (30%)**: While PCom aneurysms are the second most common (30%) and classically present with CN III palsy and ipsilateral fixed dilated pupil, they do not account for the highest proportion of aneurysmal SAH. This is a common distractor because PCom aneurysms are clinically memorable. - **Middle cerebral artery bifurcation (22%)**: MCA bifurcation aneurysms are the third most common site (22%) and often present with focal deficits, but they are less frequent than AComm aneurysms overall. - **Basilar artery tip (5%)**: Basilar tip aneurysms are rare (5%) and are associated with higher morbidity and mortality due to their deep location and proximity to brainstem structures. They are not the most common site. **High-Yield:** AComm (35%) > PCom (30%) > MCA bifurcation (22%) > Basilar tip (5%)—memorize this hierarchy for aneurysm location prevalence in aneurysmal SAH. [cite: AHA/ASA 2023 Guidelines for the Management of Aneurysmal SAH]