A 3-day-old male neonate born to non-consanguineous parents presents with poor feeding, lethargy, and a characteristic "musty" or "mousy" odour in urine. Newborn screening shows elevated plasma phenylalanine (>20 mg/dL; normal <2 mg/dL) and elevated urinary phenylketones. The infant's parents are phenotypically normal. Physical examination reveals fair skin and blonde hair. What is the primary enzymatic defect responsible for this clinical presentation?

Explanation

## Diagnosis: Phenylketonuria (PKU) ### Clinical Presentation The vignette describes classic **phenylketonuria (PKU)**, an autosomal recessive disorder of phenylalanine metabolism. The key diagnostic clues are: - Elevated plasma phenylalanine (>20 mg/dL) - Elevated urinary phenylketones (phenylpyruvate, phenyllactate, phenylacetate) - Characteristic "musty" or "mousy" odour (due to phenylacetate) - Fair skin and blonde hair (hypopigmentation from competitive inhibition of tyrosine uptake into melanocytes) - Early neurological symptoms (lethargy, poor feeding) ### Biochemical Mechanism **Key Point:** Phenylalanine hydroxylase catalyzes the conversion of phenylalanine → tyrosine using tetrahydrofolate (BH₄) as a cofactor. ```mermaid flowchart TD A["Phenylalanine (dietary)"]:::outcome --> B{"Phenylalanine hydroxylase<br/>+ BH₄ cofactor"}:::decision B -->|"Normal enzyme"|C["Tyrosine (for catecholamines,<br/>melanin, proteins)"]:::outcome B -->|"Enzyme deficiency<br/>PKU"|D["Phenylalanine accumulates"]:::urgent D --> E["Phenylpyruvate<br/>Phenyllactate<br/>Phenylacetate"]:::outcome E --> F["Musty odour<br/>Neurological damage<br/>Hypopigmentation"]:::urgent ``` ### Why Phenylalanine Hydroxylase Deficiency? Deficiency of this enzyme prevents conversion of phenylalanine to tyrosine, causing: 1. **Accumulation of phenylalanine** and its transamination products (phenylpyruvate, phenyllactate) 2. **Tyrosine becomes conditionally essential** (normally non-essential, but now required from diet) 3. **Neurological toxicity** from excess phenylalanine and its metabolites (competitive inhibition of neutral amino acid transporters, interference with monoamine synthesis) 4. **Hypopigmentation** from reduced melanin synthesis (tyrosine is the precursor for melanin) **High-Yield:** PKU is one of the few preventable causes of intellectual disability if detected early via newborn screening and treated with a phenylalanine-restricted diet. ### Management - **Newborn screening** (tandem mass spectrometry or fluorometry) - **Dietary restriction** of phenylalanine (special low-phe formulas and foods) - **Lifelong monitoring** of plasma phenylalanine levels - **Maternal PKU management** in pregnancy (to prevent fetal damage) **Clinical Pearl:** Even heterozygous carriers (parents) have normal phenotypes because they retain ~50% enzyme activity, which is sufficient for normal phenylalanine metabolism.