A 63-year-old woman has noticed weakness in her right hand such that she is having difficulty writing and doing up buttons. On examination, there is marked atrophy of the forearm and hand muscles with fasciculations. Tone is increased and reflexes are brisk. Sensory testing in the hand is normal. There are also fasciculations on her thighs, which she has never noticed before.For the above patient with muscle weakness, select the most likely anatomic site for the disorder