A 45-year-old man presents to the clinic for evaluation of weakness in his arms and legs. The symptoms started gradually, and are now more noticeable and interfering with his ability to work as an electrician. On physical examination, the cranial nerves are normal, but there is weakness of his left handgrip and right leg quadriceps with loss of muscle bulk. There are obvious fasciculations over the left forearm and right thigh. Tone is increased in the arms and legs and the reflexes are brisk. Which of the following is the most likely diagnosis?