A 62 years old man presents with a 6-month history of generalized fatigue and weakness. He initially noted difficulty ascending stairs and walking prolonged distances. Over the past several months, he has also developed intermittent dysarthria, difficulty raising his arms above his head, and impotence. On examination, prominent weakness in the pelvic and shoulder girdles with mild distal leg weakness is noted. Reflexes are initially absent, although they are obtainable after a brief period of exercise. Sensation and coordination are normal. What is the most likely diagnosis?