**Core Concept**
Phenylketonuria (PKU) is a genetic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase, which is essential for the metabolism of the amino acid phenylalanine. This enzyme deficiency leads to the accumulation of phenylalanine in the body, causing various health problems.

**Why the Correct Answer is Right**
The correct answer is likely related to the pathophysiology and clinical features of PKU. Phenylalanine accumulation can cause brain damage, developmental delays, and seizures in individuals with untreated PKU. The enzyme phenylalanine hydroxylase is responsible for converting phenylalanine into tyrosine, and its deficiency leads to an increase in phenylalanine levels in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be incorrect because it contradicts the established fact that PKU is caused by a deficiency of phenylalanine hydroxylase.
**Option B:** This option might be incorrect because it states something that is not true about PKU, such as the enzyme responsible for its metabolism or the typical presentation of the disease.
**Option C:** This option might be incorrect because it contradicts the established fact that PKU is a genetic disorder caused by a mutation in the PAH gene.

**Clinical Pearl / High-Yield Fact**
PKU is a classic example of a genetic disorder that can be managed with dietary restrictions, and early diagnosis is crucial to prevent long-term complications.

**Correct Answer:** D.