A 70-year-old man is experiencing neck pain, progressive weakness, and numbness in his arms and legs without bowel or bladder dysfunction or dysphagia. Upon examination, he has atrophy of his upper extremities but normal muscle bulk in his legs. Strength is diffusely 4/5 throughout. Cervical spine radiographs show spondylosis. Electromyography (EMG) reveals fibrillations with increased amplitude in the extensor carpi radialis and pronator teres. Nerve conduction studies demonstrate slowing conduction diffusely in the sural, peroneal, and ulnar nerves, and severe slowing in the median nerve. Testing of the tongue and thoracic paraspinal muscles does not show fibrillations or positive short waves. What is the most likely diagnosis?