What is the primary cause of phenylketonuria (PKU) in a neonate?

Phenylketonuria (PKU) in a neonate is primarily caused by an inborn error of metabolism, specifically a deficiency in the enzyme phenylalanine hydroxylase. This enzyme is necessary for the conversion of phenylalanine, an amino acid found in many protein-containing foods, into tyrosine. When this conversion is impaired due to the enzyme deficiency, phenylalanine accumulates to toxic levels in the body, which can lead to serious neurological damage if not identified and managed early.

Since PKU is a genetic condition, the inborn error directly ties back to the genetic mutations affecting the production or function of the phenylalanine hydroxylase enzyme. Although genetic predisposition plays a role, it is the metabolic error that defines the condition and leads to the clinical manifestations observed in neonates diagnosed with PKU.

The other options, while relevant to healthcare and development, do not directly cause PKU: environmental factors and inadequate prenatal care do not stem from the metabolic dysfunction that characterizes PKU, and while genetic predisposition is related, it is the metabolic error that is the defining characteristic of the disorder.