Defect in phenylketonuria-
**Core Concept**
Phenylketonuria (PKU) is a genetic disorder caused by a defect in the metabolism of the amino acid phenylalanine (Phe). It is characterized by the inability to break down Phe, leading to its accumulation in the body.
**Why the Correct Answer is Right**
The defect in PKU is a mutation in the PAH gene, which encodes for the enzyme phenylalanine hydroxylase (PAH). PAH is responsible for converting Phe into tyrosine through a hydroxylation reaction. The mutation leads to a deficiency of PAH activity, resulting in the accumulation of Phe and its metabolites in the body. This can cause brain damage, intellectual disability, and other complications if left untreated.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the defect in PKU.
* **Option B:** This option refers to a different genetic disorder, not related to PKU.
* **Option C:** This option is incorrect because it does not describe the primary defect in PKU.
**Clinical Pearl / High-Yield Fact**
Early diagnosis and treatment of PKU through dietary restriction of Phe can prevent brain damage and other complications. It is essential for newborns to undergo a screening test for PKU to detect this condition early.
**Correct Answer: D. Deficiency of phenylalanine hydroxylase (PAH) enzyme.**