Phenylketonuria (PKU): Amino Acid Metabolism Defect
| Primary Defect: | Phenylalanine Hydroxylase (PAH) (98% of cases) |
| Secondary Defect: | Dihydrobiopterin Reductase (BH4 cofactor deficiency) |
| Essential Amino Acid Shift: | Tyrosine (Becomes conditionally essential) |
| Pathognomonic Odor: | Musty or Mousy Odor (From phenylacetate excretion) |
High-Yield Core Realities:
- The Alternative Pathway Shunt: When the conversion of phenylalanine to tyrosine is blocked, phenylalanine accumulates in tissues and shifts into alternative transamination pathways. It converts into toxic metabolites: phenylpyruvate, phenyllactate, and phenylacetate, which dump into the urine.
- Clinical Phenotype Presentation: Untreated infants present within a few months of birth with profound intellectual disability, severe seizures, growth restriction, and eczema. They exhibit characteristic fair skin, blonde hair, and blue eyes due to a downstream melanin synthesis block (tyrosine is the direct precursor to melanin).
- The Screening Window Rationale: Neonatal screening is performed via heel prick 2-3 days after birth. Testing too early can yield a false negative because maternal enzymes clear phenylalanine in utero. The infant must undergo adequate protein feeding (breast milk or formula) for 24-48 hours to allow endogenous phenylalanine levels to rise and trigger detection.
- Maternal PKU Syndrome: A pregnant woman with poorly controlled PKU passes high levels of phenylalanine across the placenta to the fetus. Phenylalanine acts as a potent teratogen, inducing microcephaly, severe congenital heart defects (e.g., tetralogy of Fallot), and intrauterine growth restriction in the offspring, regardless of the fetus’s actual genetic status.
- Malignant PKU (BH4 Deficiency Variant): If the underlying defect is in dihydrobiopterin reductase or BH4 synthesis rather than the PAH enzyme itself, standard dietary phenylalanine restriction is insufficient. Because BH4 is an essential cofactor for both tyrosine hydroxylase and tryptophan hydroxylase, its deficiency completely halts downstream synthesis of dopamine, epinephrine, norepinephrine, and serotonin, leading to severe, progressive neurological decline despite normal phenylalanine levels. Treatment requires dietary BH4 supplementation (sapropterin) and neurotransmitter precursors (L-DOPA, 5-HTP).