**Core Concept**
Phenylketonuria (PKU) is a genetic disorder characterized by the inability to break down the amino acid **phenylalanine**. This condition is caused by a defect in the **phenylalanine hydroxylase** enzyme. The underlying principle involves the metabolism of **phenylalanine** to **tyrosine**.

**Why the Correct Answer is Right**
The correct answer is related to the enzyme responsible for converting **phenylalanine** to **tyrosine**. This enzyme is **phenylalanine hydroxylase**, which requires **tetrahydrobiopterin** as a cofactor. Deficiency of **phenylalanine hydroxylase** leads to accumulation of **phenylalanine**, causing PKU.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the enzyme involved in PKU is not related to **tryptophan** metabolism.
**Option B:** This option is incorrect as it is not directly related to **phenylalanine** metabolism.
**Option D:** This option is incorrect because it does not involve the specific enzyme deficient in PKU.

**Clinical Pearl / High-Yield Fact**
PKU is a classic example of a **genetic disorder** that can be managed with **dietary restrictions**, specifically limiting **phenylalanine** intake. Early diagnosis through **newborn screening** is crucial.

**Correct Answer:** C. Phenylalanine hydroxylase