## Core Concept
Phenylketonuria (PKU) is a genetic disorder characterized by the inability to break down phenylalanine (Phe), an amino acid found in many protein-containing foods and some artificial sweeteners. This condition results from mutations in the gene that encodes for the enzyme phenylalanine hydroxylase (PAH), which is necessary for converting phenylalanine into tyrosine. Without sufficient PAH activity, phenylalanine accumulates and can lead to intellectual disability and other serious health problems if not treated.

## Why the Correct Answer is Right
The first line of therapy for PKU involves a strict dietary regimen aimed at reducing phenylalanine intake. This dietary approach limits the consumption of phenylalanine-containing foods, effectively lowering blood phenylalanine levels to near-normal ranges. This management strategy is crucial for preventing the neurological damage associated with high levels of phenylalanine. The goal is to maintain blood phenylalanine levels within a target range, which varies by age and individual patient needs.

## Why Each Wrong Option is Incorrect
- **Option A:** While certain medications and supplements might be used as adjunct therapies in the management of PKU, such as tetrahydrobiopterin (BH4) for some patients responsive to it, they are not considered the first line of therapy.
- **Option B:** This option is not specified, but if it implies a treatment other than dietary restriction, it would be incorrect for the same reasons as Option A.
- **Option D:** Similarly, without specification, if this option suggests a treatment other than dietary management, it would be incorrect.

## Clinical Pearl / High-Yield Fact
A critical clinical pearl for PKU management is that early diagnosis and initiation of dietary therapy are crucial for preventing intellectual disability. The target blood phenylalanine levels are age-dependent, and maintaining levels within these targets significantly improves outcomes. Additionally, some patients with PKU may benefit from sapropterin dihydrochloride, a synthetic form of BH4, which can help reduce phenylalanine levels in responsive individuals.

## Correct Answer: B. Dietary restriction of phenylalanine.