Loss of previously attained developmental milestones in an infant is often associated with central nervous system tumor, neuromuscular disease, or an inborn metabolic error. An infant with developmental regression and involuntary movements who was found to be vitamin B12 deficient on the basis of unrecognized maternal vitamin B12 deficiency is described. The infant had a dramatic neurologic recovery after receiving vitamin B12. The case and a review of similar cases is presented.

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