Phenylketonuria is detected by which urine test?

NORCET-7 Mains-2024

Biochemistry & Nutrition

Easy

Appeared in: NORCET-7 Mains-2024

Explanation

The Ferric chloride test is a urine-based chemical test used to detect phenylketones.
In Phenylketonuria (PKU), excess phenylalanine is converted to phenylpyruvic acid (a phenylketone), which is excreted in the urine.
The reaction between ferric chloride and phenylpyruvic acid in the urine produces a transient blue-green color, confirming the presence of the metabolite.

Option A: The Guthrie test is a screening test for PKU, but it uses a blood sample obtained via a heel prick, not a urine sample.
Option B: The sodium nitroprusside test (Rothera's test) is used to detect ketone bodies (acetoacetate and acetone) or cystine in the urine, not the specific phenylketones found in PKU.
Option C: The term 'Blot test' is a general category for molecular biology techniques (like Western, Southern, or Northern blots) used to identify macromolecules like proteins or nucleic acids. It is not a specific urine test for PKU.

Visual 1: Diagram - A flowchart comparing the Guthrie test (blood) and Ferric chloride test (urine) for PKU screening, highlighting the different sample types and principles.
Visual 2: Image - A picture showing the color change (transient blue-green) of a positive Ferric chloride test for PKU.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Biochemical tests for Phenylketonuria (PKU) as background academic context rather than a clinical decision trigger.
Early detection of PKU through newborn screening is critical to prevent severe intellectual disability, seizures, and other neurological problems.
While the Ferric chloride test is historically significant, modern newborn screening relies on blood tests like tandem mass spectrometry, which are more sensitive and allow for earlier detection and treatment initiation.
A nurse's role includes ensuring that newborn screening is performed, educating parents about the importance of the test, and managing the care of an infant diagnosed with PKU, which primarily involves a strict low-phenylalanine diet.

How to Approach the Question

First, identify the key components of the question: the condition (Phenylketonuria) and the required specimen type (urine test).
Evaluate each option based on these two criteria.
Recall or look up the standard tests for PKU. You might know the Guthrie test is for PKU.
Critically evaluate the Guthrie test: it is for PKU, but it is a blood test. The question specifically asks for a urine test, so this option is incorrect.
Analyze the remaining options. The Ferric chloride test is a known chemical test for metabolites in urine. The sodium nitroprusside test is for different types of ketones. 'Blot test' is too general.
Conclude that the Ferric chloride test is the only option that fits the criteria of being a urine test for PKU.

Concept Tested & Keywords

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