A 2-year-old girl is brought to the physician because of failure to thrive. She was diagnosed with phenylketonuria on a routine neonatal screening. Her weight and height are currently well below normal for her age. She is following a low-phenylalanine diet. Her plasma phenylalanine concentration is within the reference range. The most appropriate next step is measurement of the concentration of which of the following in the plasma?

Question

Quizplus · Accepted Answer

Tyrosine is a non-essential amino acid that becomes essential in individuals with phenylketonuria (PKU) because phenylalanine cannot be converted to tyrosine due to the deficiency of the enzyme phenylalanine hydroxylase. Therefore, measuring tyrosine levels is important to ensure adequate intake and prevent deficiency, which can contribute to growth issues. A deficiency in phenylalanine hydroxylase (PAH) is the cause of PKU. Tyrosine is produced in cells by hydroxylating the essential amino acid phenylalanine and this reaction is catalyzed by PAH. Half of the phenylalanine required goes into the production of tyrosine; if the diet is rich in tyrosine itself, the requirements for phenylalanine are reduced by about 50%. Since this patient is on a diet restricting phenylalanine, it is necessary to periodically assay for serum tyrosine levels to ensure there is adequate amounts in the diet

A 2-Year-Old Girl Is Brought to the Physician Because of Failure