A new born infant is diagnosed with phenylketonuria. The enzyme that is deficient is
💡 Explanation
## **Core Concept**
Phenylketonuria (PKU) is a genetic disorder characterized by the inability to break down the amino acid phenylalanine (Phe). This condition results from a deficiency in an enzyme crucial for phenylalanine metabolism. The disorder leads to the accumulation of phenylalanine in the body, which can cause intellectual disability and other serious health problems if not treated.
## **Why the Correct Answer is Right**
The correct answer involves the enzyme **phenylalanine hydroxylase**. This enzyme is essential for converting phenylalanine into tyrosine, another amino acid. The reaction involves the hydroxylation of phenylalanine to form tyrosine, a process that requires tetrahydrobiopterin (BH4) as a cofactor. In PKU, due to the deficiency of phenylalanine hydroxylase, phenylalanine accumulates, leading to neurotoxicity.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not correspond to the enzyme deficient in PKU.
- **Option B:** This option is incorrect as it does not accurately represent the enzyme involved in the pathogenesis of PKU.
- **Option C:** This option might seem plausible but is incorrect because it does not specifically relate to the primary enzymatic deficiency seen in PKU.
## **Clinical Pearl / High-Yield Fact**
A critical aspect of PKU management is early diagnosis through newborn screening and dietary restriction of phenylalanine. If not treated early, PKU can lead to severe intellectual disability. A classic clinical correlation is that untreated PKU can result in "mousy" odor of the urine due to the accumulation of phenylalanine metabolites.