All the following are true about Phenylketonuria except?

SJH Nursing Officer - 2019

Biochemistry & Nutrition

Medium

Appeared in: SJH Nursing Officer - 2019

Explanation

This statement is the correct answer because it is false. Increased urinary excretion of p-hydroxyphenyl pyruvic acid is not a feature of Phenylketonuria (PKU).
In PKU, the enzyme Phenylalanine Hydroxylase is deficient, which blocks the conversion of Phenylalanine to Tyrosine.
As a result, Tyrosine levels are normal or low, and therefore its downstream metabolites, like p-hydroxyphenyl pyruvic acid, are not produced in excess.
The characteristic urinary findings in PKU are elevated levels of phenylalanine metabolites, such as phenylpyruvate, phenyllactate, and phenylacetate.
Increased excretion of p-hydroxyphenyl pyruvic acid is a hallmark of a different group of disorders known as Tyrosinemia.

Option A: This statement is a true and defining characteristic of classic Phenylketonuria. The question asks for the exception.
Option B: This statement is a true and severe consequence of untreated PKU due to the neurotoxic effects of high phenylalanine levels. The question asks for the exception.
Option D: This statement is a true pathophysiological consequence of PKU. High phenylalanine levels competitively inhibit the transport of tryptophan (the precursor to serotonin) across the blood-brain barrier, leading to reduced serotonin synthesis in the brain. The question asks for the exception.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Pathophysiology and biochemical features of Phenylketonuria (PKU) as background academic context rather than a clinical decision trigger.
Early detection of PKU through newborn screening (e.g., Guthrie test) is critical to prevent irreversible brain damage.
The cornerstone of nursing management is educating the family about lifelong dietary restrictions. This includes a diet low in phenylalanine (avoiding high-protein foods like meat, milk, eggs) and supplementing with special medical formulas that provide tyrosine and other essential nutrients.
Nurses must monitor the patient's growth, development, and blood phenylalanine levels to ensure the diet is effective and to prevent nutritional deficiencies.

How to Approach the Question

First, identify the question type. This is an 'except' question, which means you are looking for the statement that is FALSE or does not apply to the topic.
The topic is Phenylketonuria (PKU). Recall the core facts about PKU: its cause, its main consequence if untreated, and its key biochemical markers.
Evaluate each option systematically:
Option A: Is PKU caused by PAH deficiency? Yes, this is the definition. So, this is true.
Option B: Does untreated PKU cause mental retardation? Yes, this is the most feared complication. So, this is true.
Option D: Does PKU affect serotonin? Yes, high phenylalanine interferes with tryptophan transport, reducing serotonin. So, this is true.

Concept Tested & Keywords

Concept Tested: Pathophysiology and biochemical features of Phenylketonuria (PKU)
Stem keywords: Phenylketonuria, true
Lead-in keywords: except
Negative lead-in flag: EXCEPT

QawCos4XGl4qrr0d3L4DDI

E6 Nelson Textbook of Pediatrics(2024) — Volume 1 p. 856-858

E6 Biochemistry U Satyanarayana— Part 1 (pp 26-420 of 840) p. 348-350