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The appropriate management for an 18-month-old boy with musty smelling urine and developmental delays is dietary restriction of phenylalanine. This scenario suggests the possibility of phenylketonuria (PKU), which is a genetic disorder characterized by an inability to properly metabolize phenylalanine, an amino acid found in many protein-containing foods. When phenylalanine accumulates in the body due to the defect in the enzyme phenylalanine hydroxylase, it can lead to severe intellectual disability and developmental delays, as well as the characteristic musty odor in urine.

In the management of PKU, adopting a diet that restricts phenylalanine is essential. This typically involves avoiding high-protein foods such as meat, eggs, and dairy, which are rich in phenylalanine, while allowing for a special low-phenylalanine diet supplemented by a formula that provides necessary nutrients without the harmful amino acid.

Other dietary restrictions, though important in different metabolic conditions, do not apply in this case. For example, while lactose and galactose are restricted in galactosemia, they are not relevant for PKU. Supplementation of vitamin C does not relate to the management of any amino acid disorder associated with