A 3-week-old male neonate born to non-consanguineous parents presents with poor feeding, irritability, and a musty/mousy odour on the nappy. Newborn screening by tandem mass spectrometry shows elevated plasma phenylalanine (>20 mg/dL) and elevated urinary phenylketones. The infant has no developmental delay or seizures yet. What is the most appropriate immediate next step in management?

Explanation

## Clinical Context: Phenylketonuria (PKU) This neonate presents with the classic biochemical and clinical features of **classical PKU** — elevated plasma phenylalanine, phenylketones in urine, and the pathognomonic musty/mousy odour (due to phenylacetate in sweat and urine). ## Management Principles in PKU **Key Point:** Early initiation of dietary restriction (within the first 2–4 weeks of life) is the **gold standard** for preventing intellectual disability, seizures, and neuropsychiatric complications in PKU. **High-Yield:** The critical window for neuroprotection is **before 3 months of age**. Delays in starting treatment result in irreversible intellectual disability. ### Why Immediate Dietary Restriction? 1. **Phenylalanine is neurotoxic** — high levels inhibit large neutral amino acid transporters (LAT) at the blood–brain barrier, reducing tyrosine and tryptophan uptake. 2. **Neuronal damage occurs rapidly** — accumulation of phenylacetate and phenyllactate impairs myelin synthesis and synaptic function. 3. **Confirmed diagnosis** — tandem MS is the gold standard; repeat testing delays critical intervention. ### Management Algorithm ```mermaid flowchart TD A[Elevated Phe on newborn screening]:::outcome --> B{Clinical presentation + confirmation?}:::decision B -->|Yes, PKU confirmed| C[Start phenylalanine-restricted diet immediately]:::action C --> D[Refer to metabolic genetics/dietetics]:::action D --> E[Monitor plasma Phe levels weekly]:::action E --> F{Target Phe 2-6 mg/dL achieved?}:::decision F -->|Yes| G[Continue diet, long-term follow-up]:::outcome F -->|No| H[Adjust diet composition, consider BH4 testing]:::action H --> I{BH4-responsive PKU?}:::decision I -->|Yes| J[Add sapropterin to diet]:::action I -->|No| K[Continue strict diet alone]:::action ``` **Clinical Pearl:** Sapropterin (BH4) is a **second-line agent** used only after dietary control is established AND BH4 responsiveness is confirmed (via a trial of BH4 with measurement of plasma Phe response). It is NOT the first-line intervention. ## Why the Correct Answer is Best **Immediate dietary restriction + referral** addresses the time-critical neurotoxic threat and engages specialist support for ongoing monitoring and adjustment. This is the **standard of care** in all major guidelines (AAP, ACMG, European PKU guidelines). ## Table: PKU vs. Other Aminoacidurias | Feature | PKU | Alkaptonuria | Maple Syrup Urine Disease | | --- | --- | --- | --- | | **Enzyme defect** | Phenylalanine hydroxylase | Homogentisate oxidase | BCAA dehydrogenase | | **Elevated amino acid** | Phenylalanine | Homogentisic acid | Leucine, isoleucine, valine | | **Neonatal presentation** | Musty odour, poor feeding | None (asymptomatic in infancy) | Seizures, encephalopathy, sweet odour | | **Immediate action** | Dietary restriction | Observation | ICU admission, BCAA-free nutrition | | **Age of onset** | Weeks (if untreated) | Adulthood (ochronosis) | Days (neurological crisis) |