Q. A 3-year-old boy presents to the pediatric clinic with a history of delayed milestones. His mother reports that he began to sit and crawl later than most babies and, even now, he doesn't walk. He has also been having recurrent respiratory infections. On physical examination, he appears small for his age with noticeable bowing of the legs. There is a palpable lump on either side of his chest, just lateral to the sternum. His mother mentions that the boy was exclusively breastfed for the first year of life and they are strict vegans. The family recently moved to the US from a northern European country. The boy was not given any vitamin supplementation. What is the deficient vitamin in this child's case?