Review Question - QID 214805

A 12-year-old boy is brought to the clinic by his mother for frequent falls. The mother reports that ever since a “bad cold” 4 months ago, the patient has been increasingly clumsy with falls almost every day. The patient reports feeling weak in his legs but denies any fevers, chills, paresthesias, pain, or incontinence. The patient was recently diagnosed with type 1 diabetes mellitus managed with an insulin pump but is otherwise healthy. His temperature is 98.6°F (37.0°C), blood pressure is 115/70 mmHg, pulse is 65/min, and respirations are 15/min. A physical examination demonstrates a crescendo-decrescendo, mid-systolic murmur at the left sternal border. A neurological examination reveals decreased patellar and Achilles deep tendon reflexes (DTRs) bilaterally. There is also decreased motor strength and vibratory sensation in the lower extremities. Which of the following is the most likely cause of this patient’s symptoms?