Phenylketonuria is detected by which urine test

OSSSC Nursing Officer-2023

Biochemistry & Nutrition

Easy

Appeared in: OSSSC Nursing Officer-2023

Explanation

The Ferric chloride test is a qualitative urine test used to detect the presence of phenylpyruvic acid, a metabolite found in the urine of individuals with Phenylketonuria (PKU).
In this test, a few drops of ferric chloride solution are added to a urine sample. A positive result is indicated by the appearance of a transient blue-green or dark green color.
This color change occurs because the ferric ions (Fe³⁺) react with the enol form of the phenylpyruvic acid ketone group.
While historically used, this test is less sensitive than modern blood screening methods and is only positive after significant accumulation of phenylalanine metabolites.

Option A: This is a blood test, not a urine test. It involves collecting a blood sample via a heel prick onto filter paper.
Option B: This test detects ketone bodies (acetoacetate and acetone) or cystine, not the phenylketones characteristic of PKU.
Option C: This is a general term for molecular biology techniques (e.g., Western blot, Southern blot) used to detect specific proteins or DNA sequences, not a simple chemical urine test.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Urine-based diagnostic testing for Phenylketonuria (PKU) as background academic context rather than a clinical decision trigger.
While the Ferric chloride test is historically significant, modern nursing practice relies on newborn blood screening (NBS) programs using tandem mass spectrometry for early and accurate PKU detection.
A positive screening test requires immediate follow-up and confirmation. Nurses play a key role in educating parents about the importance of the test and the necessity of lifelong dietary management if the diagnosis is confirmed.
What if? If a child from a region without newborn screening presents with developmental delay, seizures, and a 'mousy' odor, a urine ferric chloride test could be a rapid, low-cost initial test to suggest PKU, prompting definitive blood testing.

How to Approach the Question

First, identify the key terms in the question: 'Phenylketonuria' (the condition) and 'urine test' (the sample type).
Second, recall the standard diagnostic and screening tests associated with PKU.
Third, differentiate the tests based on the sample required (blood vs. urine). The question specifically asks for a urine test.
Evaluate the options: The Guthrie test is a well-known PKU test, but it uses blood. The Sodium nitroprusside test is for different metabolites. A 'Blot test' is a lab technique, not a specific urine test for PKU metabolites.
This leaves the Ferric chloride test, which is known to react with phenylketones in urine, making it the correct answer.

Concept Tested & Keywords

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