**Core Concept**
Phenylketonuria (PKU) is a genetic disorder characterized by the inability to break down the amino acid phenylalanine (Phe). This condition is due to a deficiency in the enzyme responsible for converting Phe into another amino acid.

**Why the Correct Answer is Right**
The correct answer is the enzyme phenylalanine hydroxylase (PAH), which is crucial for the conversion of Phe to tyrosine. PAH requires tetrahydrobiopterin (BH4) as a cofactor to facilitate this reaction. In PKU, mutations in the PAH gene lead to reduced or absent enzyme activity, resulting in the accumulation of toxic levels of Phe in the body.

**Why Each Wrong Option is Incorrect**
* **Option A:** Tyrosine aminotransferase is an enzyme involved in the breakdown of tyrosine, but it is not directly involved in the conversion of Phe to tyrosine.
* **Option B:** Histidase is an enzyme involved in the catabolism of the amino acid histidine, which is unrelated to Phe metabolism.
* **Option C:** This option is blank, so we will move on to the next one.

**Clinical Pearl / High-Yield Fact**
The classic symptoms of untreated PKU include developmental delay, intellectual disability, and seizures. Early diagnosis and treatment with a Phe-restricted diet can prevent these complications and improve outcomes.

**Correct Answer: C. Phenylalanine hydroxylase**