A 9-month-old girl presents with her parents to the pediatrician for a well child visit. The newborn screening panel for the girl indicated an absence of phenylalanine hydroxylase. The child is doing well and the parents indicate no concerns. The child is meeting all developmental milestones. Vital signs are within normal limits, and physical examination reveals no abnormalities or overt manifestations of the disease process. The mother indicates they have been compliant with the prescribed formula and inquires about starting solid foods.
Which of the following amino acids is essential in her condition and should be recommended for supplementation after weening from formula?
- A) Cysteine
- B) Isoleucine
- C) Phenylalanine
- D) Tyrosine
- E) Valine
This question is describing phenylketonuria, a condition characterized by the absence of the phenylalanine hydroxylase enzyme. Phenylalanine hydroxylase is responsible for the conversion of phenylalanine to tyrosine. In its absence, patients lose the ability to endogenously synthesize tyrosine from phenylalanine, making it an essential amino acid that must be increased in the diet. Phenylalanine cannot be broken down and builds up in the system, leading to deleterious side effects such as neurologic injury and a mousy body odor; thus, it is restricted in the diet of affected individuals. (Choice D).Answer choice A: Cysteine, is incorrect. Cysteine is a required supplement in homocystinuria, a condition associated with absent or reduced cystathione synthase enzyme activity, leading to buildup of methionine and homocysteine.
Answer choice B: Isoleucine, is incorrect. Maple syrup urine disease, in which branched chain amino acids cannot be broken down due to a lack of the branched chain alpha-ketoacid dehydrogenase complex (BCKDC), results in elevation of the branched-chain amino acids (isoleucine, leucine, and valine). Patients will present with ketonuria, causing a characteristic maple syrup smell of their urine (Choices B & E). It is alco characterized by psychomotor delay and feeding problemsAnswer choice C: Phenylalanine, is incorrect. Because phenylalanine cannot broken down in patients with phenylketonuria, it should not be supplemented in the diet.
Answer choice E: Valine, is incorrect. As noted above, valine, isoleucine, and leucine build up in patients with maple syrup urine disease.
Key Learning Point
Phenylketonuria (PKU) presents with an absence of phenylalanine hydroxylase, preventing the conversion of phenylalanine to tyrosine. It may present with a variety of symptoms, including neurologic injury, fair skin (due to impaired melanin production), and a musty body odor. Patients are instructed to avoid phenylalanine in their diets and supplement tyrosine.
Dr. Ted O'Connell