 Timing is essential, as some of the side effects of vitamin B12 deficiency are reversible (such as red blood cell (RBC) indices, peripheral RBC smear findings such as hypersegmented neutrophils, or even high levels of methylmalonyl CoA), but some side effects are irreversible as they are of a neurological source (such as tabes dorsalis, and peripheral neuropathy).High suspicion should be exercised when a neonate, or a pediatric patient presents with anemia, protein in the urine, sufficient vitamin B12 dietary intake, and no signs of pernicious anemia.This is a rare disease with prevalence about 1 in 200,000 to 1 in 600,000.