A 7-year-old girl is brought to the clinic for evaluation of developmental delays. She has been walking since she was 18 months old, but has been losing the ability to speak. Her parents also report that she has difficulty with coordination, has experienced episodes of excessive hand wringing, and has lost the ability to use her hands since the age of 5 years. She has not had any medical problems in the past, and there is no family history of neurological or developmental disorders. On physical examination, she has decreased muscle tone and coordination and is unable to perform voluntary movements, but she is not in any distress. The remainder of the physical examination is unremarkable. Which of the following is the most likely diagnosis?