**Core Concept**
Pegvaliase is a pharmacological treatment for phenylketonuria (PKU), a rare genetic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase (PAH). This enzyme is necessary for the breakdown of the amino acid phenylalanine (Phe).

**Why the Correct Answer is Right**
Pegvaliase is an enzyme replacement therapy that works by replacing the deficient PAH enzyme in individuals with PKU. By doing so, it allows for the breakdown of phenylalanine, thereby reducing its accumulation in the body. This treatment is specifically approved for adults with PKU who have a confirmed diagnosis of hyperphenylalaninemia (elevated levels of phenylalanine in the blood).

**Why Each Wrong Option is Incorrect**
**Option A:** Pegvaliase is not approved for the treatment of type 1 diabetes mellitus; this condition is typically managed with insulin therapy.
**Option B:** Although pegvaliase is a treatment for a genetic disorder, it is not used for the treatment of cystic fibrosis, which involves a different set of underlying pathophysiological mechanisms.
**Option C:** Pegvaliase is not approved for the treatment of phenylketonuria in children; its safety and efficacy in pediatric populations have not been established.

**Clinical Pearl / High-Yield Fact**
Pegvaliase is administered via subcutaneous injection, and its use is associated with potential side effects such as injection-site reactions, nausea, and diarrhea.

**Correct Answer:** C. Pegvaliase is approved for adults with phenylketonuria.