What does a positive PKU screening indicate about an infant's metabolic ability?

A positive Phenylketonuria (PKU) screening indicates that an infant has a deficiency in the enzyme phenylalanine hydroxylase, which is essential for the metabolism of the amino acid phenylalanine. When this enzyme is not functioning properly, phenylalanine accumulates in the body, leading to potential neurological damage and other health issues.

The correct response highlights that the infant has an inability to effectively process essential amino acids, specifically phenylalanine. In a healthy individual, phenylalanine is converted into tyrosine by the enzyme. When this conversion does not occur due to the enzyme deficiency, the metabolism of this essential amino acid is impaired, leading to toxic levels in the blood. This highlights the importance of early detection through PKU screening, allowing for dietary modifications to prevent serious health complications.

Each of the other options relates to different metabolic processes that are not associated with PKU. They do not accurately reflect the specific dysfunction caused by the lack of phenylalanine hydroxylase, which is why they do not pertain to what a positive PKU screening indicates about an infant's metabolic ability.