**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 systemic complications.

**Why the Correct Answer is Right**
The correct answer involves the identification of the enzyme responsible for the metabolism of phenylalanine. Phenylalanine hydroxylase (PAH) catalyzes the conversion of phenylalanine to tyrosine. In PKU, mutations in the PAH gene result in reduced or absent PAH enzyme activity, leading to the accumulation of phenylalanine and its toxic metabolites. This accumulation can cause brain damage, intellectual disability, and other systemic complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct enzyme involved in phenylalanine metabolism. While other enzymes, such as aminotransferases, may be involved in amino acid metabolism, they are not directly responsible for the defect in PKU.
**Option B:** This option is incorrect because it is not a recognized enzyme involved in phenylalanine metabolism. Other enzymes, such as tyrosine hydroxylase, are involved in the metabolism of tyrosine, but not phenylalanine.
**Option C:** This option is incorrect because it is not a specific enzyme involved in phenylalanine metabolism. While the liver is involved in the metabolism of many substances, including amino acids, this option is too nonspecific to be correct.
**Option D:** This option is incorrect because it is not a recognized enzyme involved in phenylalanine metabolism. Other enzymes, such as histidine decarboxylase, are involved in the metabolism of other amino acids, but not phenylalanine.

**Clinical Pearl / High-Yield Fact**
PKU is a classic example of a genetic disorder caused by a deficiency of a single enzyme. Understanding the role of this enzyme in phenylalanine metabolism is crucial for diagnosis and management of the condition. Early detection and treatment can prevent long-term complications and improve outcomes for individuals with PKU.

**Correct Answer:** A. Phenylalanine hydroxylase.