Q. A 35-year-old male underwent a surgical resection of his ileum and ascending colon five years ago following a major motor vehicle accident. Recently, he has been experiencing progressive numbness, tingling in his lower limbs, and difficulty walking and maintaining balance. He denies any history of back pain or injury. On physical exam, he has normal power, difficulty with joint position sensation, gait ataxia, and brisk and pathologic lower extremity reflexes. Given the patient's surgical history and current neurological symptoms, which of the following conditions might have developed in this patient over the past five years?