PLE-6873

PLE-6873

A 72-year-old man comes to the physician because of a 7-month history of leg weakness and dry eyes and mouth. He also has had a 10.4-kg (23-lb) weight loss over the past 4 months, despite no change in appetite. He has smoked one and a half packs of cigarettes daily for 50 years and drinks 4 oz of alcohol daily. He has peptic ulcer disease and emphysema. His medications include cimetidine, theophylline, and low-dose prednisone. Examination shows mild ptosis and a barrel-shaped chest. Breath sounds are distant. There is moderate weakness of the proximal muscles of the lower extremities, with absent reflexes. He has difficulty rising from a chair. Sensory examination shows no abnormalities. An x-ray film shows a hyperinflated chest and a 3 x 4-cm mass in the right hilum. The neurologic findings are most likely due to a lesion involving which of the following?