A 76-year-old woman is brought to the emergency department due to sudden loss of right-sided motor function in her face, arm, and leg. She has a past medical history of hypertension, hypercholesterolemia, and previous transient ischemic attack, and was recently placed on Coumadin for new-onset atrial fibrillation. A computed tomography (CT) scan is performed and is shown. Which of the following neurologic structures is most commonly involved in this patient's injury?