A 5-month-old infant is admitted with vomiting and eczema over the lower extremities. The mother states that the infant is not developing as fast as her other children. When a diagnosis of phenylketonuria (PKU) is made, the nurse should recognize which of the following as the basic underlying defect involved?

Choice A rationale The absence of the enzyme hexosaminidase component A is the underlying defect in Tay-Sachs disease, a lysosomal storage disorder, not phenylketonuria (PKU). Tay-Sachs causes a buildup of GM2 ganglioside lipids in neurons, leading to progressive neurological deterioration, blindness, and spasticity, with symptoms typically presenting later than the described 5-month-old's developmental delay. Choice B rationale A chromosomal abnormality involving pairs 21 to 23 describes the genetic basis of Down syndrome (Trisomy 21) and other common trisomies, not PKU. Down syndrome is a set of physical and mental characteristics caused by an extra full or partial copy of chromosome 21, and its presentation is fundamentally different from the metabolic defect of PKU. Choice C rationale The absence of surfactant (a phospholipid-protein complex) in the lungs is the primary cause of Respiratory Distress Syndrome (RDS) in premature newborns. Surfactant reduces alveolar surface tension, preventing lung collapse. This defect is a pulmonary issue and bears no relation to the systemic metabolic disorder that affects the processing of the amino acid phenylalanine. Choice D rationale Phenylketonuria (PKU) is an autosomal recessive metabolic disorder caused by a deficiency of the liver enzyme phenylalanine hydroxylase (PAH). This enzyme normally converts the essential amino acid phenylalanine into tyrosine. Without adequate PAH, phenylalanine accumulates in the blood and brain, becoming toxic to the central nervous system, leading to the reported vomiting, eczema, and profound intellectual disability and developmental delay if untreated.