Q. A 42 year-old man presents with a several-month course of weakness that began in the left arm and has since spread to both arms and legs. Neurological exam reveals: - asymmetric but diffuse upper and lower extremity weakness; - mixed hyperreflexia and areflexia throughout the bilateral upper and lower extremities; - bilateral pathologic (ie, positive) Babinski's; - sensory exam is normal. Which of the following is the most likely diagnosis?