A 67-year-old male patient arrives in the emergency department complaining of progressively worsening bouts of right leg weakness. His wife notes that he is confused and often has trouble forming words at these times, which typically last about a day. The right leg weakness improves only slightly from instance to instance, leaving him weaker each time. Because the symptoms are transient, he has put off seeing a physician until now. His wife is worried that he might be having a stroke. He reports having smoked a pack of cigarettes each day for 45 years.