A 60 years old man presents with increased incidence of stumbling and loss of balance, which has developed gradually over the last year. The patient reports associated muscle twitching and painful muscle cramps in the legs, and has also noticed he occasionally has difficulty swallowing. The neurologic examination is pertinent for bilateral lower-extremity weakness, more severe on the right, with foot intrinsic muscles, diffuse fasciculations, and hyperreflexia, with deep tendon reflexes being brisker on the right lower extremity, and a positive right Babinski sign. Sensation is preserved throughout. What is the most likely diagnosis?