A 47-year-old man presents with a 13-year history of weakness and atrophy of the right hand muscles. Thoracic outlet syndrome surgery did not help. CK level and CSF examination were normal. Cervical MRI was normal. NCS showed no conduction block. The CMAPs of the right median and ulnar nerves were reduced. Sensory responses were normal. EMG revealed diffuse chronic denervation of the right arm muscles.
At what level is the lesion likely located?
1. Motor neurons
2. Motor nerves
3. Brachial plexus