**Question:** Phenylketonuria is -

A. A deficiency in phenylalanine hydroxylase enzyme
B. An increase in phenylalanine levels in the blood
C. A decrease in tyrosine synthesis
D. An increase in tyrosine levels in the blood

**Correct Answer:** B. An increase in phenylalanine levels in the blood

**Core Concept:** Phenylketonuria (PKU) is a genetic disorder characterized by a deficiency in phenylalanine hydroxylase enzyme (A), which is responsible for converting phenylalanine to tyrosine. This deficiency leads to a buildup of phenylalanine (B) in the blood, which can cause severe neurological damage if left untreated.

**Why the Correct Answer is Right:** Phenylalanine hydroxylase deficiency results in an inability to properly break down phenylalanine, an essential amino acid that plays a crucial role in protein synthesis. Elevated levels of phenylalanine in the blood (option B) can lead to toxic effects on the developing brain, causing developmental delays, intellectual disability, and seizures.

**Why Each Wrong Option is Incorrect:**

Option A incorrectly states that PKU is associated with a decrease in tyrosine synthesis, which is not the primary issue in the disease.

Option C is incorrect because PKU does not primarily affect tyrosine synthesis but rather phenylalanine levels. Tyrosine is synthesized from phenylalanine, so a decrease in tyrosine would not be expected in PKU.

Option D is incorrect because PKU primarily results in elevated phenylalanine levels (option B), not tyrosine levels. Elevated tyrosine levels would be expected in a deficiency of tyrosine hydroxylase, which is not the case in PKU.

**Clinical Pearl:** Phenylalanine hydroxylase deficiency should be suspected in neonates presenting with neurological symptoms and elevated plasma phenylalanine levels. Early diagnosis and treatment with a low-phenylalanine diet can help prevent neurological deficits and improve outcomes in affected individuals.