Anatomy clinical correlates Quiz: Ace Your Exams

A 68-year-old man is brought to the emergency department after sustaining a traumatic head injury in a motor vehicle collision. The patient is unresponsive, and the paramedics report a witnessed generalized-tonic-clonic seizure. Past history is significant for mitral valve replacement four years ago, hypertension, and coronary artery disease. Current medications include aspirin, warfarin, enalapril, metoprolol, and atorvastatin. Temperature is 37.3°C (99.1°F), pulse is 59/min, respirations are 7/min and irregular, and blood pressure is 170/92 mm Hg. On examination, the patient appears comatose with mispositioned and fixed pupils. The upper extremities are adducted, extended, and in hyperpronation. The lower extremities appear extended, adducted, and internally rotated with the feet in plantar flexion. A non-contrast CT scan of the head demonstrates extensive intraparenchymal hemorrhage with a horizontal shift of midline structures. Damage to which of the following anatomic structures most likely contributed to the extensor posturing observed in this patient?