Review Question - QID 210747

A patient presents to the emergency department with severe pain in her head. She states that the pain started 1 hour ago after rugby practice. Typically, she takes a special medication for bad headaches, but this time she took 2 doses and noticed no improvement in her symptoms. The patient's current medications that she can recall include sumatriptan and naproxen. She has been admitted to the hospital several times recently for panic attacks and suicide attempts. Her temperature is 99.2°F (37.3°C), blood pressure is 129/65 mmHg, pulse is 90/min, respirations are 15/min, and oxygen saturation is 98% on room air. The patient is given dihydroergotamine, IV fluids, and is started on 100% oxygen. 2 hours later, the patient demonstrates slurring of her speech. On physical exam, there is drooping of the left side of her mouth, and her left upper extremity demonstrates 2/5 strength. A head CT is ordered. Which of the following is the most likely cause of this patient's current presentation?