A 62-year-old man, who has not seen a physician in 20 years, presents to your clinic with a burning sensation in his feet. The symptoms have been progressing slowly over the last 6 months. There is no associated motor weakness or skin changes. He reports no significant past medical history and takes no medications. His alcohol intake is minimal. On examination, he has reduced pinprick/vibration sensation and proprioception in the ankles with absent ankle reflexes. Which one of the following blood tests would you expect to be abnormal?
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