**Core Concept**
Phenylketonuria (PKU) is a genetic disorder caused by a deficiency of an enzyme involved in the breakdown of the amino acid phenylalanine. This enzyme deficiency leads to the accumulation of phenylalanine and its metabolites in the body, resulting in various neurological and developmental problems.

**Why the Correct Answer is Right**
The enzyme deficient in PKU is phenylalanine hydroxylase (PAH). PAH is a crucial enzyme in the metabolism of phenylalanine, converting it into tyrosine. In individuals with PKU, mutations in the PAH gene lead to a deficiency of the enzyme, causing phenylalanine to accumulate in the blood and tissues. This accumulation is toxic to the brain and other organs, resulting in the characteristic symptoms of PKU.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a relevant enzyme in the context of PKU metabolism.
**Option B:** Homogentisate 1,2-dioxygenase is an enzyme involved in tyrosine metabolism, not phenylalanine metabolism.
**Option C:** Aldehyde dehydrogenase is an enzyme involved in the metabolism of various aldehydes, not phenylalanine or tyrosine.

**Clinical Pearl / High-Yield Fact**
PKU is an autosomal recessive disorder, meaning that individuals must inherit two defective copies of the PAH gene (one from each parent) to develop the condition.

**Correct Answer:** C. Phenylalanine hydroxylase.