A nurse is assessing a 2-year-old toddler who was recently diagnosed with phenylketonuria (PKU). Which assessment finding would be most characteristic of this condition?

1 Jaundice and hepatomegaly
2 Musty odor to urine and skin ✓ 정답
3 Excessive weight gain and lethargy
4 Frequent vomiting and diarrhea

해설

Musty odor to urine and skin is the most characteristic finding in PKU, resulting from phenylalanine metabolite accumulation. Other options like jaundice/hepatomegaly, excessive weight gain/lethargy, and vomiting/diarrhea are not specific to PKU.

같은 주제 다음 문제A nurse is assessing a 3-month-old infant who was recently diagnosed with phenylketonuria …

심화 해설

Understanding the Pathophysiology

Phenylketonuria (PKU), also known as phenylalanine hydroxylase (PAH) deficiency, is one of the earliest recognized inborn errors of metabolism. In this condition, the body lacks a functional enzyme needed to break down the amino acid phenylalanine. When this pathway is blocked, phenylalanine accumulates in the blood and tissues. The body then shunts the excess phenylalanine into alternative metabolic pathways, leading to the production of abnormal phenylalanine metabolites ^[1]. These metabolites, particularly phenylacetic acid, are excreted in the urine and sweat, producing a characteristic musty or mousy odor. This odor is a direct clinical consequence of the biochemical defect and is a hallmark sign of untreated or poorly managed PKU.

Analysis of Answer Options

The correct answer is Option 2. The musty odor to urine and skin is the most characteristic assessment finding for PKU among the choices provided. This distinctive odor results directly from the accumulation and excretion of abnormal phenylalanine metabolites ^[1].

Why Other Options Are Incorrect

Option 1 (Jaundice and hepatomegaly): These findings are more indicative of liver dysfunction or metabolic disorders affecting bilirubin processing, such as galactosemia or tyrosinemia. While the liver is involved in phenylalanine metabolism, hepatomegaly and jaundice are not classic presenting signs of PKU.

Option 3 (Excessive weight gain and lethargy): Infants with untreated PKU may initially have normal weight gain, but as toxic metabolite levels rise, they typically develop neurological symptoms like irritability, seizures, and developmental delay. Lethargy can occur, but excessive weight gain is not a characteristic feature.

Option 4 (Frequent vomiting and diarrhea): While gastrointestinal disturbances can occur in various inborn errors of metabolism, they are not the hallmark of PKU. The primary pathology in PKU is the neurotoxic effect of elevated phenylalanine, not a primary gastrointestinal process.

Clinical Application and NCLEX-RN Focus

For the NCLEX-RN, it is essential to connect the underlying genetic defect to its observable clinical manifestation. PKU serves as the paradigm for newborn screening because early detection and dietary intervention can prevent severe intellectual disability. The development of a bacterial inhibition assay in the 1960s, which allowed for mass screening, was a landmark achievement built on the foundational observations of abnormal phenylalanine metabolites in affected children ^[1]. When assessing a toddler with a new PKU diagnosis, the nurse should recognize that the musty odor is a classic sign of the disease's biochemical activity. This assessment finding is a direct clue that phenylalanine levels are elevated and that the dietary management plan needs to be strictly evaluated and reinforced.

References (research sources)

[1]
Phenylketonuria (PKU)Research articleDaley SF, Corado A. (2026)

임상 시나리오

Clinical Assessment of Phenylketonuria (PKU)

When assessing a toddler with suspected or diagnosed PKU, the nurse should focus on signs of phenylalanine accumulation. The most distinctive clinical finding is a musty or mousy odor emanating from the urine, skin, and sweat. This odor is caused by the excretion of phenylacetic acid, a byproduct of alternative phenylalanine metabolism.

Other potential findings in untreated children include fair skin and hair (due to melanin synthesis disruption), eczematous rash, microcephaly, and developmental delays. However, the musty odor remains the most specific physical assessment cue.

Nursing Actions

Document the presence and characteristics of any unusual body or urine odor immediately.
Review newborn screening results and dietary adherence history.
Monitor growth parameters and neurodevelopmental milestones closely.
Reinforce the importance of strict dietary restriction of phenylalanine to prevent neurotoxic effects.

핵심 개념

Phenylketonuria (PKU) — An autosomal recessive inborn error of metabolism caused by a deficiency of phenylalanine hydroxylase, leading to accumulation of phenylalanine.
Phenylalanine Hydroxylase (PAH) — The liver enzyme that converts the essential amino acid phenylalanine to tyrosine; deficient in PKU.
Phenylacetic Acid — An abnormal metabolite of phenylalanine that accumulates in PKU and is excreted in urine and sweat, causing a musty odor.
Musty Odor — A distinctive mousy or musty smell of the urine, skin, and sweat, considered a hallmark clinical sign of untreated PKU.
Inborn Error of Metabolism — A genetic disorder resulting in a block in a metabolic pathway due to an enzyme deficiency, often presenting in infancy or early childhood.

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A nurse is assessing a 2-year-old toddler who was recently diagnosed with phenylketonuria (PKU). Which assessment finding would be most characteristic of this condition?

심화 해설

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핵심 개념

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