Review Question - QID 214273

A 51-year-old woman presents to her primary care physician with a 1-month history of muscle weakness. She started noticing that she was dropping things more frequently over the last 3 months, but she became concerned when she started feeling weaker over the last month. Her past medical history is significant for a gastric bypass surgery complicated by necrosis of a segment of bowel. She says that since the surgery she has been having fatty, foul-smelling stools. Physical exam reveals ataxia, decreased deep tendon reflexes, diffuse myalgias, and conjunctival pallor. No rashes are noted, and she denies pain anywhere else. Histology shows a normocytic anemia with no abnormalities in other cell lines. The substance that is most likely deficient in this patient has which of the following functions?