A 69-year-old radiologist develops an acute intense headache combined with slurring of the speech and confusion. In the emergency department, he is found to have a Glasgow coma scale of 7. An emergent CT scan of the brain reveals a well-demarcated site with bleeding in the left frontal lobe. The bleeding appears to be centered in the cortex. The patient has no other pertinent history and does not take any medications. Which of the following is most likely responsible for his findings?