The growth curve shown demonstrates poor growth over the 6 months of this child's life. Homocystinuria is an autosomal recessive metabolic disease caused by deficiencies of cystathionine b-synthase, methylenetetrahydrofolate reductase, or the coenzyme for N5-methyltetrahydrofolate methyltransferase. Manifestations include poor growth, arachnodactyly, osteoporosis, dislocated lenses, and mental retardation. In addition, thromboembolic phenomena may be seen in the pulmonary and systemic arteries and particularly in the cerebral vasculature; vascular occlusive disease is, in turn, one of the many causes of acute infantile hemiplegia. None of the other disorders listed in the question is associated with acute hemiplegia. Phenylketonuria causes retardation and, on occasion, seizures; maple syrup urine disease, an abnormality of the metabolism of leucine, leads to seizures and rapid deterioration of the CNS in newborn infants; and histidinemia and cystathioninuria are most likely a benign aminoaciduria with no effect on the CNS. Many states now include these diseases in their newborn screening programs.