A child is noted to be severely mentally retarded. Physical examination reveals a pot-bellied, pale child with a puffy face. The child's tongue is enlarged. Dietary deficiency of which of the following substances can produce this pattern?

The disease is cretinism, characterized by a profound lack of thyroid hormone in a developing child, leading to mental retardation and the physical findings described in the question stem. Cretinism can be due to dietary deficiency of iodine (now rare in this country because of iodized salt), to developmental failure of thyroid formation, or to a defect in thyroxine synthesis. Calcium deficiency in children can cause osteoporosis or osteopenia. Iron deficiency can cause a hypochromic, microcytic anemia. Magnesium deficiency is uncommon, but can cause decreased reflexes, and blunts the parathyroid response to hypocalcemia.