A 42-year-old woman presents to the emergency depament at 8 PM, mildly somnolent and complaining of the "worst headache of her life," which began at 6 AM on the same day, awakening her. She took acetaminophen (Tylenol) twice during the day, with some relief. At noon she staed to have nausea with vomiting, and by 3 PM she had developed right arm and leg weakness. She denies any head trauma. Which of the following is the most likely diagnosis