A 55-year-old woman comes to the physician because of progressive weakness in both her legs over the past 6 months. She has had trouble arising from a chair; she finds it easier after rocking back and forth first. She has had a 20-lb weight loss during this time. She has smoked one pack of cigarettes daily for 30 years. Examination shows weakness of hip flexion. Sensory examination shows no abnormalities. An x-ray of the chest shows a nodular density. Which of the following is the most likely location of the abnormality?
Anterior horns
Muscle
Myoneural junction
Spinal cord
Superior cervical ganglion
