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?

Vitamin A is important in maintaining epithelial surfaces. Deficiency of this vitamin can lead to squamous metaplasia of respiratory epithelium, predisposing to infection. Increased keratin buildup leads to follicular plugging and papular dermatosis. Desquamated keratinaceous debris in the urinary tract forms the nidus for stones. Ocular complications of vitamin A deficiency include xerophthalmia and corneal scarring, which can lead to blindness. Vitamin B1 (thiamine) deficiency causes problems such as Wernicke disease, neuropathy, and cardiomyopathy. Vitamin D deficiency in children causes rickets, characterized by bone deformities. Vitamin E deficiency occurs rarely; it causes neurologic symptoms related to degeneration of the axons in the posterior columns of the spinal cord. Vitamin K deficiency can result in a bleeding diathesis.