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Quizzes > Health & Medicine

PKU NCLEX Quiz: What's True of Phenylketonuria?

Quick, free quiz with NCLEX questions on PKU. Instant feedback and short rationales.

Editorial: Review CompletedCreated By: Charo PovedanoUpdated Aug 26, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration of quiz sheets and amino acid structure on a golden yellow background for NCLEX PKU practice

This quiz helps you check your knowledge of phenylketonuria for the NCLEX, with clear questions and instant feedback. You will practice diet limits, newborn screening basics, and the PAH enzyme defect in simple steps. For related practice, try the genetic disorders quiz and build metabolic skills with diabetes nclex questions.

What enzyme is deficient in classic phenylketonuria (PKU)?
Phenylalanine hydroxylase
Dihydropteridine reductase
Phenylalanine synthase
Tyrosine hydroxylase
Phenylalanine hydroxylase (PAH) catalyzes the conversion of phenylalanine to tyrosine, and its deficiency is the hallmark of classic PKU. Without functional PAH, phenylalanine accumulates to toxic levels. Early detection and dietary management are crucial to prevent neurological damage.
What is the inheritance pattern of phenylketonuria?
Mitochondrial inheritance
Autosomal dominant
X-linked recessive
Autosomal recessive
PKU follows an autosomal recessive inheritance pattern, meaning two mutated copies of the PAH gene are required for disease manifestation. Carriers typically do not show symptoms. Genetic counseling is recommended for families with a history of PKU.
Which amino acid accumulates in patients with untreated PKU?
Tyrosine
Methionine
Phenylalanine
Leucine
In PKU, PAH deficiency prevents the conversion of phenylalanine to tyrosine, causing phenylalanine to accumulate. Elevated phenylalanine levels are neurotoxic if not managed. Monitoring blood phenylalanine is key to treatment.
Newborn screening for PKU is most commonly performed using which sample type?
Hair follicle
Urine sample
Saliva swab
Heel-prick blood spot
The heel-prick blood spot test (Guthrie test or tandem mass spectrometry) is the standard newborn screening method for PKU. It allows early detection before symptoms develop. Early dietary intervention can then begin promptly.
Untreated PKU leads to a deficiency of which amino acid?
Tryptophan
Histidine
Tyrosine
Lysine
Because PAH converts phenylalanine to tyrosine, PAH deficiency in PKU reduces tyrosine synthesis, making tyrosine conditionally essential. Low tyrosine can impair neurotransmitter synthesis. Supplementation of tyrosine is part of dietary management.
A classic clinical sign of untreated PKU is:
Blue sclerae
Cherry-red spot in the macula
Purpura formation
Musty odor of urine
Accumulation of phenylalanine and its metabolites gives PKU patients a characteristic musty or 'mouse-like' odor in urine. This odor is due to phenylacetate and phenylacetylglutamine. It is an early clinical clue before neurological signs appear.
Which of the following is a mainstay of dietary management in PKU?
High-phenylalanine diet
No dietary restrictions
Low-phenylalanine diet
High-protein diet
Dietary restriction of phenylalanine is essential to prevent neurotoxic buildup in PKU. Patients consume medical formulas low in phenylalanine and avoid high-protein foods. Adherence to diet from infancy preserves cognitive development.
Which artificial sweetener must be avoided by individuals with PKU?
Stevia
Saccharin
Aspartame
Sucralose
Aspartame is metabolized into phenylalanine, which can exacerbate PKU if ingested. Food labels in many countries carry a phenylketonuric warning for aspartame content. Patients must avoid products containing aspartame.
Why is tyrosine considered a conditionally essential amino acid in PKU?
Because tyrosine is produced from tryptophan via the indole pathway
Because dietary tyrosine is metabolized back to phenylalanine
Because PAH converts phenylalanine to tyrosine and this pathway is blocked in PKU
Because BH4 is required for tyrosine absorption
PAH catalyzes the hydroxylation of phenylalanine to form tyrosine. In PKU, PAH deficiency halts this step, so tyrosine must be supplied in the diet. Hence tyrosine is conditionally essential for individuals with PKU.
Maternal PKU syndrome is most commonly associated with which fetal outcome?
Limb reduction anomalies without cardiac issues
Neural tube defects
Microcephaly and congenital heart defects
Hydrocephalus
High maternal phenylalanine levels are teratogenic, leading to fetal microcephaly, heart defects, and low birth weight. Strict metabolic control before and during pregnancy reduces these risks. This condition is known as maternal PKU syndrome.
Sapropterin dihydrochloride operates as a synthetic form of which cofactor?
Riboflavin
Biotin
Tetrahydrobiopterin (BH4)
Folic acid
Sapropterin is an orally active synthetic form of BH4, a cofactor for PAH. It can enhance residual PAH activity in some PKU patients. BH4 responsiveness testing identifies candidates for this therapy.
The PAH gene responsible for PKU is located on which chromosome?
Chromosome X
Chromosome 6
Chromosome 12
Chromosome 11
The PAH gene maps to chromosome 12q23.2. Mutations in this gene impair phenylalanine metabolism. More than 500 pathogenic variants have been identified.
Which phenylketone metabolite is neurotoxic and accumulates in PKU?
Phenylbutyrate
Phenylacetate
Phenylpyruvate
Phenylpropionate
Phenylpyruvate is formed from transamination of excess phenylalanine and is neurotoxic. Its accumulation contributes to the characteristic musty odor and to neurologic symptoms. Other phenylketones may also accumulate but phenylpyruvate is primary.
What is the recommended target plasma phenylalanine level for children under 12 with PKU?
1000 to 1200 µmol/L
20 to 50 µmol/L
500 to 800 µmol/L
120 to 360 µmol/L
Guidelines recommend maintaining plasma phenylalanine levels between 120 and 360 µmol/L in children under 12 to optimize neurodevelopment. Levels above this range correlate with cognitive deficits. Regular monitoring guides dietary adjustments.
Which historical test first enabled newborn PKU screening?
Tandem mass spectrometry for BH4
Guthrie bacterial inhibition assay
Western blot for PAH protein
ELISA for PAH
The Guthrie test uses a bacterial inhibition assay to detect elevated phenylalanine in blood spots. Introduced in the 1960s, it dramatically improved early PKU diagnosis. Modern programs often use tandem mass spectrometry, but Guthrie remains historically significant.
An elevated ratio of phenylalanine to tyrosine in plasma primarily indicates:
Excess dietary tyrosine intake
Overproduction of tyrosine
Impaired conversion of phenylalanine to tyrosine
Impaired tyrosine transamination
In PKU, blockage of PAH activity leads to excess phenylalanine and relative tyrosine deficiency, raising the Phe:Tyr ratio. This ratio is a diagnostic and monitoring parameter. It reflects metabolic control efficacy.
Which variant of hyperphenylalaninemia is characterized by normal PAH activity but defective BH4 recycling?
Tyrosine hydroxylase deficiency
Phenylalanine hydroxylase deficiency
Dihydropteridine reductase deficiency
GTP cyclohydrolase I deficiency
Dihydropteridine reductase (DHPR) is required to regenerate BH4 from quinonoid dihydrobiopterin. DHPR deficiency impairs BH4 recycling, reducing PAH function despite normal enzyme structure. This form of hyperphenylalaninemia responds to BH4 and neurotransmitter precursors.
Neurological damage in PKU is primarily due to:
High phenylalanine competing with other large neutral amino acids at the blood-brain barrier
Autoimmune response against tyrosine hydroxylase
Excess melanin production depleting neurotransmitters
Deposition of phenylalanine crystals in neurons
Elevated phenylalanine saturates the LAT1 transporter at the blood-brain barrier, reducing uptake of other large neutral amino acids essential for neurotransmitter synthesis. This imbalance leads to impaired brain development. Direct toxicity also plays a role but transport competition is pivotal.
Which transporter mediates phenylalanine transport across the blood-brain barrier?
GLUT1
EAAT2
SLC6A1
L-type amino acid transporter 1 (LAT1)
LAT1 (SLC7A5) is the primary large neutral amino acid transporter at the blood-brain barrier, including phenylalanine. Overloading LAT1 with phenylalanine impairs uptake of other amino acids. LAT1 function is central to PKU neuropathology.
The most common type of mutation in the PAH gene causing PKU is:
Large deletion
Frameshift mutation
Nonsense mutation
Missense mutation
Missense mutations, which replace one amino acid with another, account for the majority of PAH gene defects in PKU. These point mutations often reduce enzyme activity. Genotype analysis guides prognosis and therapy responsiveness.
Supplementation with large neutral amino acids in PKU works by:
Increasing BH4 availability
Enhancing PAH enzyme expression
Reducing phenylalanine absorption in the gut
Competing with phenylalanine for brain uptake
Large neutral amino acid (LNAA) supplements flood the LAT1 transporter at the blood-brain barrier, reducing phenylalanine influx into the CNS. This strategy lowers cerebral phenylalanine levels despite peripheral hyperphenylalaninemia. LNAAs also provide essential substrates for neurotransmitter synthesis.
Untreated PKU leads to hypomyelination because:
High levels of BH4 inhibit oligodendrocyte function
Excess phenylalanine directly demyelinates neurons
Reduced tyrosine availability impairs myelin protein synthesis
Autoantibodies against myelin basic protein form
Tyrosine is a precursor for melanin and catecholamines but also contributes to myelin protein production. Low tyrosine levels in PKU impair synthesis of myelin components, leading to white matter deficits. Hypomyelination underlies many neurological symptoms.
Tetrahydrobiopterin (BH4) is a cofactor for all the following enzymes EXCEPT:
Phenylalanine aldolase
Tyrosine hydroxylase
Tryptophan hydroxylase
Phenylalanine hydroxylase
BH4 acts as a cofactor for aromatic amino acid hydroxylases - including PAH, tyrosine hydroxylase, and tryptophan hydroxylase - but not for phenylalanine aldolase, which is not a physiological enzyme in phenylalanine metabolism. This specificity underlies BH4 therapy.
The R408W mutation in the PAH gene most commonly results in:
Increased BH4 binding affinity
A truncated protein lacking the catalytic domain
Enhanced PAH enzymatic activity
A misfolded enzyme retained in the endoplasmic reticulum
The R408W missense mutation is prevalent in European PKU patients and leads to misfolding of PAH, causing its retention and degradation in the endoplasmic reticulum. This severely reduces functional enzyme levels. It is a common target for genotype - phenotype correlation studies.
Allele-specific PCR for neonatal PKU screening is primarily used to detect:
PAH mRNA expression levels
BH4 deficiency disorders
Elevated levels of phenylalanine
Known point mutations in the PAH gene
Allele-specific PCR assays are designed to amplify and detect specific PAH gene variants known to cause PKU. While biochemical tests measure phenylalanine levels, PCR-based screening can confirm genotypes rapidly. This complements metabolic screening in high-risk populations.
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Study Outcomes

  1. Identify True Characteristics of PKU -

    Recognize which of the following is true of PKU by recalling key aspects of phenylketonuria pathophysiology and phenotypic presentation.

  2. Differentiate Clinical Presentation and Lab Findings -

    Distinguish common symptoms and diagnostic results associated with PKU, including elevated phenylalanine levels and related biochemical markers.

  3. Apply Dietary and Therapeutic Interventions -

    Outline appropriate low”phenylalanine diet plans and nursing interventions to manage PKU based on current NCLEX PKU questions guidelines.

  4. Analyze NCLEX-Style PKU Questions -

    Develop strategies to approach PKU NCLEX question formats effectively and improve accuracy under exam conditions.

  5. Evaluate Test-Prep Performance -

    Use instant quiz feedback to assess proficiency in phenylketonuria NCLEX practice and identify areas for further study.

Cheat Sheet

  1. Pathophysiology of PKU -

    Phenylketonuria results from a deficiency of phenylalanine hydroxylase (PAH), blocking the conversion of phenylalanine to tyrosine and leading to toxic buildup of phenylalanine in the blood and brain. Remember the simple formula: Phe --(PAH, BH₄)→ Tyr; if PAH is missing, Phe accumulates. When you see an NCLEX PKU question asking which of the following is true of PKU, first recall this core enzymatic defect.

  2. Clinical Manifestations -

    Untreated PKU often presents with intellectual disability, seizures, eczema, and a characteristic musty body odor due to excess phenylacetic acid. Use the mnemonic "MUSTY PHenylketonuria" (MUSTY = musty odor, U = unusual behavior, S = seizures, T = tremors, Y = yellowish hair/pale skin). This quick phrase helps you ace that PKU NCLEX question every time.

  3. Newborn Screening -

    All newborns undergo a heel-prick blood test 24 - 48 hours after birth to measure phenylalanine levels, enabling early diagnosis and intervention. Early detection through this phenylketonuria NCLEX practice point prevents irreversible neurotoxicity and ensures timely dietary management. Remember: screening within the first week of life is crucial for best outcomes.

  4. Dietary Management -

    A lifelong low-phenylalanine diet supplemented with tyrosine is the cornerstone of therapy, controlling plasma phenylalanine levels between 120 - 360 µmol/L. A handy tip is to track daily Phe intake with specialized formulas and low-protein foods; typical targets hover around 200 - 500 mg of Phe per day. This PKU test prep fact underpins many NCLEX PKU questions on diet and nutrition.

  5. Genetics & Counseling -

    PKU is inherited in an autosomal recessive pattern, so parents who are carriers have a 25% chance of an affected child with each pregnancy. Genetic counseling and carrier screening are recommended for at-risk families to discuss recurrence risks and prenatal testing options. Keeping this inheritance pattern in mind will help you select which of the following is true of PKU in any exam scenario.

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