**Question:** Which of the following is a feature of Phenylketonuria?

A. Elevated blood phenylalanine levels
B. Decreased urine ketone body production
C. Seizures as a presenting symptom
D. Unchanged liver enzymes

**Core Concept:** Phenylketonuria (PKU) is an inherited disorder of amino acid catabolism, primarily affecting the amino acid phenylalanine. It is caused by a deficiency in the enzyme phenylalanine hydroxylase, which is essential for converting phenylalanine to tyrosine. As a result, phenylalanine levels rise in the blood, leading to neurological damage if left untreated.

**Why the Correct Answer is Right:** Elevated blood phenylalanine levels (Option A) is the hallmark feature of PKU due to the impaired ability to catabolize phenylalanine. This, if left untreated, can lead to mental retardation and seizures in children. Untreated PKU has been a major cause of severe mental retardation in the past, but with early diagnosis and proper dietary intervention, the prognosis has improved significantly.

**Why Each Wrong Option is Incorrect:**

B. Decreased urine ketone body production (Option B) is incorrect because PKU leads to elevated blood phenylalanine levels, not decreased ketone body production. Ketone bodies are formed when fatty acids are catabolized in the liver, and PKU does not affect this process.

C. Seizures as a presenting symptom (Option C) is not always the case in PKU diagnosis. PKU symptoms can vary in presentation, sometimes leading to developmental delays or behavioral issues. Seizures may be present but are not the sole presenting symptom. Early diagnosis is essential for timely intervention and improved outcomes.

D. Unchanged liver enzymes (Option D) is incorrect because PKU results in elevated blood phenylalanine levels due to impaired phenylalanine catabolism. Liver enzymes are not directly affected by this condition, so option D is not a representative feature of PKU.

**Clinical Pearl:** Early diagnosis of PKU is crucial for preventing severe neurological complications. A newborn screening test is the standard method used to detect PKU, where elevated phenylalanine levels in the newborn's blood sample indicate a potential PKU diagnosis. Proper dietary management, including a low-phenylalanine diet, can significantly improve the neurological outcome and overall health of affected individuals.