A 65-year-old man comes to the physician because of progressive weakness in both legs for the past 6 months. He has difficulty standing up from a chair and going up stairs. He finds it easier to stand up from a chair if he rocks back and forth first. He has also had a 7-kg weight loss during this time. He has smoked two packs of cigarettes daily for 45 years. Physical examination shows weakness of hip flexion and knee extension. A Babinski sign is absent. A chest x-ray shows a mass in the left lower lobe. Laboratory studies show:
Erythrocyte sedimentation rate (ESR) Elevated
Creatine kinase (CK) Normal
Which of the following is the most likely location of this patient’s abnormality?
