A 55-year-old woman is brought to the hospital after collapsing suddenly, with a severe headache preceding the incident. Upon arrival at the Emergency Department, her Glasgow Coma Scale (GCS) has improved to 13, and a CT scan confirms a subarachnoid hemorrhage.
You are summoned to assess her 72 hours after admission because her GCS has progressively declined to 8 over the past hour. She is currently breathing air, and her SpO2 is at 97%.
What is the most probable cause of this secondary deterioration?