Review Question - QID 216733

A 51-year-old man presents to his primary care physician with a 3-month history of increasing weakness. He says that he has noticed weakness in his right arm and left leg while engaging in activities of daily living. For example, his left leg will spasm and give way randomly while walking. Also, his right arm will weaken very quickly while carrying a shopping bag, forcing him to transfer the bag to the other hand. His temperature is 98.3°F (36.8°C), blood pressure is 120/82 mmHg, pulse is 79/min, respirations are 14/min, and oxygen saturation is 99% on room air. On physical exam, he has atrophy of his right biceps and spasticity of his left quadriceps. He has 1+ right triceps reflex and 3+ left Achilles reflex. Which of the following is the most appropriate disease-modifying treatment for this patient's symptoms?