A 40-year-old man presents with muscle weakness. He cannot open his hand for a handshake and cannot extend his arm after flexing it. On physical examination, he has marked atrophy of leg and arm muscles, ptosis, and a fixed facial expression. There is testicular atrophy. Laboratory studies demonstrate mild diabetes. A muscle biopsy reveals atrophy of type I fibers, hyperophy of type II fibers, and numerous fibers with centrally located nuclei. Which of the following is the most likely diagnosis?