## **Core Concept**
Phenylketonuria (PKU) is a genetic disorder characterized by the inability to break down the amino acid phenylalanine (Phe), due to a deficiency in the enzyme **phenylalanine hydroxylase**. This leads to the accumulation of Phe in the body, which can cause intellectual disability, seizures, and other serious health problems if not treated.

## **Why the Correct Answer is Right**
The initial line of treatment for PKU involves a strict dietary regimen aimed at reducing phenylalanine intake. This is achieved through a **phenylalanine-restricted diet**, where the individual avoids foods high in protein and phenylalanine. Additionally, special formulas that are low in phenylalanine but provide all other necessary nutrients are used. This dietary management helps to keep phenylalanine levels within a safe range, preventing further brain damage and allowing for normal development.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain medications and supplements might be used adjunctively in the management of PKU, they are not the initial line of treatment.
- **Option B:** This option is not specified, but if it implies a different primary approach, it would be incorrect because dietary management is the cornerstone of PKU treatment.
- **Option C:** Similar to Option A, if this suggests a secondary or alternative treatment approach, it would be incorrect as the initial line of treatment.
- **Option D:** If this option does not align with dietary restrictions and management, it would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A critical aspect of PKU management is early diagnosis and intervention. Newborn screening programs often include PKU testing, allowing for early initiation of dietary restrictions. A key clinical pearl is that **adherence to the phenylalanine-restricted diet is crucial, especially during infancy and childhood, to prevent intellectual disability and other complications**.

## **Correct Answer:** D. Low phenylalanine diet.