## Why Non-contrast CT head is right The anterior communicating artery (AComm, marked **A**) is the most common site of saccular (berry) aneurysm formation in the Circle of Willis, accounting for approximately 30–35% of intracranial aneurysms. Rupture presents with thunderclap headache—sudden, severe, "worst headache of my life"—due to subarachnoid hemorrhage. Non-contrast CT head is the investigation of choice with ~95% sensitivity within the first 6 hours of symptom onset, showing blood in the subarachnoid space. This patient's presentation (thunderclap headache, neck stiffness, photophobia) is classic for subarachnoid hemorrhage from AComm aneurysm rupture. (Gray's Anatomy 42e Ch 28; Harrison 21e Ch 425) ## Why each distractor is wrong - **Lumbar puncture with CSF analysis**: LP is reserved for cases where CT is negative but clinical suspicion remains high (e.g., >6 hours post-ictus). It demonstrates xanthochromia (yellow discoloration of CSF due to hemoglobin breakdown), confirming subarachnoid hemorrhage. However, it is NOT the first-line investigation and carries risk of herniation if performed before imaging rules out mass effect. - **MR angiography of cerebral vessels**: MRA is useful for detecting and characterizing aneurysms once subarachnoid hemorrhage is confirmed on CT, but it is slower than CT and not the initial screening test in acute presentation. It is typically used after CT diagnosis is established. - **Transcranial Doppler ultrasound**: TCD is used to detect and monitor vasospasm (a complication occurring 4–14 days post-hemorrhage), not for acute diagnosis of aneurysmal rupture. **High-Yield:** AComm aneurysm rupture = thunderclap headache + subarachnoid hemorrhage → non-contrast CT head first (95% sensitivity in first 6 hours); LP with xanthochromia if CT negative. [cite: Gray's Anatomy 42e Ch 28; Harrison 21e Ch 425]
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