A 6-month-old infant from rural Maharashtra is brought to the pediatric clinic with developmental delay and a musty odor on the skin. Newborn screening was not performed. Serum phenylalanine is 1200 μmol/L (normal <120 μmol/L). The parents are consanguineous. Which of the following statements about phenylketonuria (PKU) is NOT true?

Explanation

## Phenylketonuria: Biochemistry and Clinical Features ### Enzymatic Defect **Key Point:** Phenylalanine hydroxylase (PAH) deficiency is the primary enzymatic lesion in classical PKU, preventing conversion of phenylalanine to tyrosine [cite:Robbins 10e Ch 5]. ### Mechanism of Neurological Damage **High-Yield:** Elevated phenylalanine competitively inhibits LAT1 (large neutral amino acid transporter 1), reducing brain uptake of other large neutral amino acids (leucine, isoleucine, valine, tryptophan, tyrosine). This disrupts: - Myelin formation (tyrosine is required for dopamine and norepinephrine synthesis) - Protein synthesis in the developing brain - Neurotransmitter production ### Critical Window for Prevention **Warning:** Early dietary restriction (phenylalanine-restricted diet started within the first 2 weeks of life) can **prevent or minimize** neurological damage, but cannot **completely prevent all complications** once the infant is symptomatic. Some metabolic byproducts (phenylacetate, phenyllactate) accumulate and cause transient damage even with early intervention. Additionally, if treatment is started late (after 6 months), irreversible cognitive impairment has already occurred. ### Cutaneous Manifestations **Clinical Pearl:** Hypopigmentation occurs because tyrosine is the precursor for melanin synthesis. Competitive inhibition of tyrosine uptake into melanocytes and reduced tyrosine availability both contribute. Eczema is also common in untreated PKU. ### Comparison of Amino Acidurias | Feature | PKU | Alkaptonuria | Maple Syrup Urine Disease | |---------|-----|--------------|---------------------------| | Enzyme defect | Phenylalanine hydroxylase | Homogentisate 1,2-dioxygenase | Branched-chain α-ketoacid dehydrogenase | | Elevated amino acid | Phenylalanine | Homogentisic acid | Leucine, isoleucine, valine | | Odor | Musty/mousy | None (dark urine) | Maple syrup/burnt sugar | | Neurological damage | Yes (if untreated) | No | Yes (severe, acute) | | Dark urine/ochronosis | No | Yes (alkapton) | No | | Onset | Neonatal | Adulthood (ochronosis) | Neonatal (acute metabolic crisis) | **Mnemonic:** **PAH-LAT** — **P**henylalanine **A**ccumulation via **H**ydroxylase deficiency → **L**arge neutral **A**mino acid **T**ransporter inhibition. ## Why Option 2 is Incorrect The statement claims that early dietary restriction can **completely prevent all neurological complications**. This is an overstatement. While early intervention (within 2 weeks) dramatically reduces neurological damage, some degree of transient metabolic derangement and minor neurodevelopmental effects may persist. Moreover, the question emphasizes "all" — even with perfect early treatment, subtle cognitive or behavioral effects have been documented in some cohorts. The correct statement would be: "Early dietary restriction can **prevent or significantly minimize** neurological complications." ## Summary Table: Amino Acidurias at a Glance | Disorder | Defective Enzyme | Substrate Accumulates | Product Deficient | Key Clinical Feature | |----------|-----------------|----------------------|-------------------|----------------------| | PKU | Phenylalanine hydroxylase | Phenylalanine | Tyrosine | Intellectual disability (untreated), musty odor | | Alkaptonuria | Homogentisate 1,2-dioxygenase | Homogentisic acid | Maleylacetoacetic acid | Ochronosis, arthritis (late) | | MSUD | Branched-chain α-ketoacid dehydrogenase | Leucine, isoleucine, valine | — | Acute metabolic crisis, maple syrup odor, cerebral edema | | Tyrosinemia Type I | Fumarylacetoacetate hydrolase | Tyrosine, methionine | — | Liver failure, renal tubular dysfunction, neurological crises |