Foetal infection of a pregnant woman with syphilis can be diagnosed by:

UP NHM CHO 7 Sept 2022(shift-1st)

Obstetrics & Gynaecology

Hard

Appeared in: UP NHM CHO 7 Sept 2022(shift-1st)

Explanation

The Polymerase Chain Reaction (PCR) test is the correct answer because it directly detects the genetic material (DNA) of the causative agent, Treponema pallidum.
This test is performed on amniotic fluid obtained through amniocentesis, providing direct evidence that the pathogen has crossed the placental barrier and infected the fetus.
Unlike antibody tests, PCR is not confounded by the passive transfer of maternal IgG antibodies to the fetus, making it the most definitive method for prenatal diagnosis.

Option A: ELISA is a serological test that detects antibodies. In pregnancy, maternal IgG antibodies cross the placenta. A positive ELISA in the fetus could simply reflect the mother's antibodies and does not confirm an active fetal infection.
Option B: Western blot is another type of antibody test, often used for confirmation. Like ELISA, it cannot distinguish between passively transferred maternal antibodies and a true fetal immune response.
Option C: The Coomb's test is completely unrelated to diagnosing infections. It is used to detect antibodies that are stuck to the surface of red blood cells.

Clinical Relevance

Nursing practice connection: Use the key finding related to Prenatal diagnosis of congenital syphilis to guide bedside assessment, documentation, and the next nursing action.
Accurate prenatal diagnosis of syphilis is critical because timely treatment of the pregnant woman with penicillin can effectively treat the fetus in utero, preventing the devastating effects of congenital syphilis.
Nurses play a vital role in educating pregnant women about the importance of syphilis screening and explaining the difference between maternal screening tests (like VDRL, RPR, ELISA) and definitive fetal diagnostic tests (like PCR from amniocentesis).
What if? If a pregnant woman in her third trimester is diagnosed with primary syphilis and treated, but a fetal PCR test was not done, the newborn must be proactively evaluated and potentially treated for congenital syphilis because the risk of transmission is high, and maternal treatment late in pregnancy may not be sufficient.

How to Approach the Question

First, identify the core of the question: it asks for a method to diagnose infection in the fetus, not the mother.
Consider the physiological context of pregnancy: maternal antibodies (IgG) cross the placenta. This is a key piece of information.
Evaluate each option: Ask yourself, 'What does this test detect?'
Option A (ELISA) and B (Western blot) detect antibodies. Since maternal antibodies are present in the fetus, these tests are not definitive for fetal infection.
Option C (Coomb's test) detects antibodies on red blood cells and is used for blood-related disorders, not infections like syphilis. Eliminate this as irrelevant.
Option D (PCR) detects the pathogen's DNA. This provides direct proof of the organism's presence. Since it doesn't rely on antibodies, it is not affected by maternal antibody transfer. This makes it the most accurate and definitive choice.

Concept Tested & Keywords

Concept Tested: Prenatal diagnosis of congenital syphilis
Stem keywords: Foetal infection, pregnant woman, syphilis, diagnosed
Lead-in keywords: diagnosed by
Clinical cues: Age/sex group narrows the expected diagnosis, intervention, or normal reference range.
Negative lead-in flag: false

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