Question

Emergency medical services bring a 64-year-old male to the emergency department following the sudden onset of profound room-spinning dizziness, intractable nausea, and an inability to maintain his balance. His medical records document long-standing hypertension that is refractory to multiple medications. On physical examination, blood pressure is 182/100 mm Hg and heart rate is 88/min. Neurological assessment reveals mild ptosis of the right eyelid, a right pupil that is 2 mm smaller than the left but reactive to light, and an absence of sweating on the right side of his face. Testing of sensory pathways demonstrates a marked loss of pinprick and temperature sensation over the right side of the face, as well as over the entire left side of his trunk and extremities. Motor strength is fully preserved at 5/5 in all muscle groups. Proprioception and vibratory sensation are completely intact bilaterally. Finger-to-nose testing shows pronounced dysmetria of the right upper extremity. The attending physician suspects an acute infarction involving the lateral aspect of the medulla rather than the lateral pons. Which of the following additional physical examination findings would most reliably confirm this specific anatomical diagnosis?