**Question:** Which one of the following is not a feature of Phenylketonuria?

A. Hyperphenylalaninemia
B. Homozygous or compound heterozygous mutations in the phenylalanine hydroxylase (PAH) gene
C. Symptoms typically appear within the first few weeks of life
D. Diagnosis can be confirmed with blood tests

**Core Concept:** Phenylketonuria (PKU) is a rare genetic disorder caused by mutations in the PAH gene that encodes phenylalanine hydroxylase enzyme, responsible for converting phenylalanine to tyrosine. This deficiency leads to elevated phenylalanine levels in the blood and cerebrospinal fluid, resulting in neurological damage if left untreated.

**Why the Correct Answer is Right:**

A. Hyperphenylalaninemia is a feature of PKU, as the elevated phenylalanine levels lead to a condition called hyperphenylalaninemia.

B. Diagnosis of PKU can be confirmed with blood tests, which measure phenylalanine levels. This is a correct feature of PKU.

C. PKU typically presents with symptoms appearing within the first few weeks of life, making early diagnosis and intervention crucial. This is a correct feature of PKU.

D. PKU is a rare genetic disorder caused by mutations in the PAH gene. The correct answer is right because option D states that PKU can be confirmed with blood tests, which is a feature of PKU, but not a correct answer to the question of which option is not a feature of PKU.

**Why Each Wrong Option is Incorrect:**

A. Hyperphenylalaninemia is a feature of PKU, as mentioned above.

B. While blood tests are used for diagnosis, this is not an incorrect feature of PKU; rather, it is a correct feature. Therefore, this option is incorrect because it does not address the question's focus on what is NOT a feature of PKU.

C. PKU typically presents with symptoms appearing within the first few weeks of life, making early diagnosis and intervention crucial. This is a correct feature of PKU, but not the focus of the question, which is to identify what is NOT a feature of PKU.

D. PKU is a rare genetic disorder caused by mutations in the PAH gene. This option is incorrect because it addresses the cause of PKU rather than an aspect that is NOT a feature of PKU.

**Clinical Pearl:** Early diagnosis and treatment of PKU are crucial to prevent neurological impairment due to the accumulation of phenylalanine in the body. Early diagnosis involves newborn screening, where elevated phenylalanine levels are detected in blood samples taken within the first days of life. Appropriate dietary restrictions and supplementation can prevent neurological damage in affected individuals.