Review Question - QID 109484

An 11-year-old boy presents to the pediatrician with his mother for evaluation of difficulty walking. His mother reports that the patient was walking normally until about 1 year ago when he started to complain of weakness in his legs. He seems to be less steady on his feet than before and he has fallen 2 times at home. Prior to 1 year ago, the patient had no difficulty walking and was active on his school’s soccer team. He has no other past medical history. The patient is an only child and his mother denies any family history of neurological disease. His temperature is 98°F (36.7°C), pulse is 70/min, blood pressure is 118/77 mmHg, respirations are 13/min, and oxygen saturation is 98% on room air. On physical examination, the patient has mildly slurred speech. He has a wide-based gait with symmetric weakness and decreased sensation in his lower extremities. The patient also has the physical exam findings seen in Figures A and B. Which of the following is the most likely etiology of this patient’s presentation?