A 74-year-old woman develops occipital headache, vomiting, and dizziness. She looks unwell, her blood pressure is 180/100 mm Hg, pulse is 70/min, and respirations are 30/min. She is unable to sit or walk because of unsteadiness. Over the next few hours, she develops a decline in her level of consciousness.For the above patient with altered level of consciousness, select the most likely diagnosis.