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 Hu Qi Feng
Pediatrics
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The pathogenesis of phenylketonuria

Phenylketonuria is an amino acid metabolic disease caused by a deficiency of phenylalanine hydroxylase. Phenylalanine is an essential amino acid for the human body, and it is partially converted into tyrosine by phenylalanine hydroxylase. Due to the reduced activity of phenylalanine hydroxylase, phenylalanine cannot be transformed into tyrosine. This leads to extremely high concentrations of phenylalanine in the blood, cerebrospinal fluid, and tissues. Through alternative metabolic pathways, large amounts of phenylpyruvic acid, phenylacetic acid, phenyllactic acid, and others are produced. High concentrations of phenylalanine and its metabolic products can cause brain damage.

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Written by Hu Qi Feng
Pediatrics
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Phenylketonuria Clinical Symptoms

Phenylketonuria may appear normal at birth, but symptoms usually start to appear between three to six months, with the symptoms becoming most evident at one year old. These are often manifestations of the nervous system, with prominent developmental delays in intelligence. The IQ is often lower than normal, and there may be abnormal behaviors, minor epileptic seizures, and, in a few children, increased muscle tone and hyperactive tendon reflexes. Due to insufficient melanin synthesis a few months after birth, affected children’s hair color changes from black to yellow, their skin is fair, and skin eczema is relatively common. Due to the elevated excretion of phenylacetic acid in urine and sweat, there is a distinct mouse-like urine smell.

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Written by Zeng Hai Jiang
Pediatrics
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"Phenylketonuria weakly positive" means what?

The indication of weakly positive phenylketonuria suggests a re-examination of blood phenylalanine levels. Generally, there are no obvious specialized clinical symptoms of phenylketonuria in the newborn period. Some newborns may experience feeding difficulties, vomiting, or irritability, and the manifestations of phenylketonuria gradually appear after three months of age. Currently, hospitals routinely screen newborns for phenylketonuria. This screening is conducted 72 hours after birth and must be completed by no later than 20 days post-birth. If the result of the blood phenylalanine test is weakly positive or positive, further testing is generally recommended to determine whether the infant has phenylketonuria or if other factors are affecting the phenylalanine levels.

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Written by Yan Xin Liang
Pediatrics
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Phenylketonuria Test Methods

The examination methods for phenylketonuria primarily include newborn screenings usually performed three to seven days after birth by drawing a blood sample from the heel and placing drops on specialized blood collection filter paper. Once dried, the sample is sent to a screening laboratory to determine the concentration of phenylalanine. If the phenylalanine concentration exceeds the cutoff value, further examinations and confirmatory tests are necessary, with the confirmatory test involving the measurement of phenylalanine concentration. Another method involves the analysis of urinary pterin profile, mainly used to differentiate dihydrobiopterin reductase deficiency. Additionally, gene analysis can be applied to detect mutations in genes such as phenylalanine hydroxylase, aiding in genetic diagnoses and prenatal diagnoses.

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Written by Yan Xin Liang
Pediatrics
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Phenylketonuria is what type of inheritance?

Phenylketonuria is an autosomal recessive genetic disorder and is the most common congenital amino acid metabolism disorder. The main clinical features include intellectual disability, pale skin and hair pigmentation, and a mouse-like urine smell. This condition is named after the high levels of phenylketonic acid metabolites excreted in the urine of affected patients. Phenylketonuria is an autosomal recessive genetic disease, and its primary pathological mechanism is due to the lack of phenylalanine hydroxylase, which fails to convert phenylalanine into tyrosine, leading to increased concentrations of phenylalanine in the blood, cerebrospinal fluid, and various tissues. Because the main metabolic pathway is blocked, secondary metabolic pathways are enhanced, leading to the deamination of phenylalanine and the production of large amounts of phenylketonic acid. Through oxidation, this produces byproducts such as phenylacetic acid, phenyllactic acid, and para-hydroxyphenylketonic acid, thereby resulting in a series of clinical symptoms.