All the following are true about Phenylketonuria except?

Appeared in: SJH Nursing Officer - 2019

Explanation

The statement is false because p-hydroxyphenyl pyruvic acid is a breakdown product of tyrosine.
In Phenylketonuria (PKU), the enzyme that converts phenylalanine to tyrosine is deficient.
This leads to a lack of tyrosine, and consequently, a decrease in its metabolites.
Increased excretion of p-hydroxyphenyl pyruvic acid is a hallmark of a different metabolic disorder called Tyrosinemia.

Option A: This statement is true. The primary cause of classic PKU is the deficiency of the enzyme phenylalanine hydroxylase.
Option B: This statement is true. Untreated PKU leads to the accumulation of phenylalanine in the brain, causing severe neurotoxicity and resulting in mental retardation.
Option D: This statement is true. High levels of phenylalanine competitively inhibit the transport of tryptophan (the precursor to serotonin) across the blood-brain barrier, leading to reduced serotonin synthesis.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Pathophysiology and biochemical findings in Phenylketonuria (PKU) as background academic context rather than a clinical decision trigger.
Newborn screening for PKU is mandatory in many countries, including India, as early detection and dietary intervention can prevent irreversible brain damage.
The mainstay of treatment is a lifelong diet low in phenylalanine. This requires avoiding high-protein foods and using special medical formulas.
Nurses play a crucial role in educating families about the strict dietary requirements, monitoring growth and development, and providing psychosocial support.

How to Approach the Question

First, identify that the question uses a negative frame ('except'), meaning you must find the false statement.
Recall the basic pathophysiology of Phenylketonuria (PKU): a deficiency of the enzyme phenylalanine hydroxylase.
Consider the direct consequences: Phenylalanine accumulates, and its downstream product, tyrosine, is not formed.
Evaluate each option against this core concept.
Option A (enzyme deficiency) is the definition of PKU.
Option B (mental retardation) is the classic outcome of untreated PKU.

Concept Tested & Keywords

Concept Tested: Pathophysiology and biochemical findings in Phenylketonuria (PKU)
Stem keywords: Phenylketonuria, true, except
Lead-in keywords: except
Negative lead-in flag: Question asks for the exception

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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

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Attempt every question from this paper in a timed mock, then review the full solution for each one.