A 50-year-old woman develops a sudden, severe headache and is taken to the emergency department. On examination, she has nuchal rigidity. Her blood pressure is 115/83 mm Hg. A lumbar puncture is done; the CSF shows numerous RBCs, no neutrophils, a few mononuclear cells, and a normal glucose level. The Gram stain result is negative. CT imaging shows subarachnoid hemorrhage at the base of the brain. Which of the following vascular events has most likely occurred in this woman?

About 1 in 50 individuals have a saccular (berry) aneurysm. Although this lesion is present at birth as a congenital defect in the arterial media at intracerebral arterial branch points, it can manifest later in life with aneurysmal dilation and possible rupture. These aneurysms are the most common cause of spontaneous subarachnoid hemorrhage in adults. The bleeding from amyloid angiopathy is in the peripheral cortex, and most likely to occur in association with Alzheimer's disease. A hypertensive hemorrhage from arteriolosclerosis tends to remain within the brain parenchyma. Thromboemboli can cause infarctions, most often in the distribution of the middle cerebral artery in the cortex, and embolic infarcts can be hemorrhagic, but the blood typically does not reach the CSF. A subdural hematoma over the brain surface results from a tear of bridging veins.