A newborn is found on universal metabolic screening to have elevated phenylalanine (1800 μmol/L, normal <120). The infant is placed on a phenylalanine-restricted diet. The underlying enzyme deficiency in classic PKU is:
- A Phenylalanine hydroxylase (PAH) — converting phenylalanine to tyrosine ✓
- B Phenylalanine transaminase
- C Tyrosinase
- D Homogentisate oxidase
Correct answer: A. Phenylalanine hydroxylase (PAH) — converting phenylalanine to tyrosine
Explanation
Classic phenylketonuria (PKU) is caused by deficiency of phenylalanine hydroxylase (PAH), which normally converts phenylalanine to tyrosine using tetrahydrobiopterin (BH4) as a cofactor. Accumulation of phenylalanine and its toxic metabolites (phenylpyruvic, phenylacetic, phenyllactic acids) causes progressive intellectual disability, seizures, eczema, and fair phenotype. Treatment is lifelong dietary phenylalanine restriction; BH4 (sapropterin) supplements the residual PAH activity in BH4-responsive PKU.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.