An epidemiologic study observes increased numbers of respiratory tract infections among children living in a community in which most families are at the poverty level. The infectious agents include Streptococcus pneumoniae, Haemophilus influenzae, and Klebsiella pneumoniae. Most of the children have had pneumonitis and rubeola infection. The study documents increased rates of keratomalacia, urinary tract calculi, and generalized papular dermatosis in these children as they reach adulthood. These children are most likely to have a deficiency of which of the following vitamins?