Is phenylketonuria related to a baby's face turning dark?

Written by Wu Ben Rong
Pediatrics
Updated on May 09, 2025
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Baby's dark complexion is not directly related to phenylketonuria (PKU). Phenylketonuria is a relatively common amino acid metabolism disorder caused by a deficiency in the enzyme needed for the metabolic pathway of phenylalanine, preventing it from being converted into tyrosine. This leads to the accumulation of phenylalanine and keto acids in the body, which are then excreted in large amounts through urine. Children with phenylketonuria may have lighter hair and skin color within months of birth due to insufficient melanin synthesis. Typically, their skin is very pale; therefore, if a child has darker facial skin, it is not related to phenylketonuria. This condition might be due to genetic factors or prolonged exposure to the sun among other reasons.

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Written by Wu Ben Rong
Pediatrics
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Can phenylketonuria be treated with surgery?

Phenylketonuria (PKU) in children cannot be treated surgically. PKU is an autosomal recessive genetic disorder caused by a deficiency in the enzyme needed for the metabolic pathway of phenylalanine. Typically, there are no obvious symptoms in newborns, although some children may experience non-specific symptoms such as vomiting and feeding difficulties. If no relevant treatment is pursued, the child may gradually show signs of developmental delays in movement and intellect, and the hair might change from black to yellow while the skin tends to be paler. As the child grows older, the intellectual delays become more pronounced, and in severe cases, they might develop cerebral palsy. This disease can be treated in its early stages through dietary methods, and the younger the age at which treatment begins, the better the outcomes.

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Written by Zeng Hai Jiang
Pediatrics
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What tests do parents with phenylketonuria undergo?

Parents with phenylketonuria can undergo chromosome testing. Phenylketonuria is a common amino acid metabolic disease and an autosomal recessive genetic disorder. Currently, it is mainly screened after the birth of the child by testing the blood for phenylketonuria. Phenylketonuria can also be screened prenatally before the birth of the newborn. Screening can be conducted by extracting amniotic fluid from the pregnant woman for genetic screening through amniocentesis between the 16th and 20th weeks of pregnancy. If chromosomal abnormalities are detected through amniocentesis, it indicates that the fetus carries the pathogenic gene, proving that the child will have phenylketonuria after birth. Couples carrying related genes also need to undergo prenatal screening.

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Written by Yan Xin Liang
Pediatrics
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Do children with mild phenylketonuria need treatment?

Phenylketonuria is a common amino acid metabolic disease, primarily due to the deficiency of phenylalanine hydroxylase, preventing phenylalanine from being converted into tyrosine. This leads to the accumulation of phenylalanine and its keto acids in the body, which are then excreted in large amounts in the urine. It is a relatively common genetic amino acid metabolic disorder and is inherited as an autosomal recessive trait. Even in cases of mild symptoms, treatment is still required. The main treatment is a low-phenylalanine diet, which involves dietary control, as phenylalanine is an essential amino acid for protein synthesis. Special phenylalanine-formula milk powder is fed to infants, and at this stage, it is appropriate to add some complementary foods such as starchy vegetables and fruits, focusing on low-protein foods. The goal of dietary treatment is to maintain an appropriate concentration of phenylalanine in the blood.

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Written by Yan Xin Liang
Pediatrics
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Phenylketonuria smells like mouse urine.

Phenylketonuria is a common amino acid metabolic disorder primarily caused by a deficiency of phenylalanine hydroxylase. This deficiency prevents phenylalanine from being converted to tyrosine, leading to the accumulation of phenylalanine and ketone bodies, which are then excreted in large amounts in the urine. This disease is relatively common among genetic amino acid metabolic disorders and is inherited as an autosomal recessive trait. Additionally, due to the lack of phenylalanine hydroxylase, phenylalanine is metabolized through another pathway, resulting in increased production of phenyllactic acid and phenylacetic acid, which are excreted through sweat and urine, giving off a mouse-like urine odor.

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Written by Yan Xin Liang
Pediatrics
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How is phenylketonuria tested?

Phenylketonuria is a treatable inherited metabolic disease, and early diagnosis is essential for early treatment. Currently, we have generally implemented a newborn disease screening system that requires collecting peripheral blood by pricking the heel of newborns after three days of breastfeeding, dropping the blood onto specialized filter paper and sending it to a screening laboratory for phenylalanine concentration measurement. If the concentration exceeds the confirmed value, further differential diagnosis and confirmation are needed. If treatment can begin early, especially within two to three weeks after birth, the prognosis is generally good. Normally, the concentration is less than 120 µmol/L; a fetal concentration of 1200 µmol/L would be considered mild phenylketonuria.