## **Core Concept**
Toxoplasmosis is a parasitic infection caused by *Toxoplasma gondii*, which can be transmitted from mother to fetus during pregnancy. The diagnosis of congenital toxoplasmosis involves detecting specific antibodies or antigens in the fetus or newborn. Serologic tests play a crucial role in confirming the infection.

## **Why the Correct Answer is Right**
The correct answer involves detecting **IgM antibodies** against *Toxoplasma gondii*. IgM antibodies are the first line of defense and are produced early in the course of an infection. In the context of congenital toxoplasmosis, the presence of IgM antibodies in the fetus or newborn indicates a recent infection, as IgM antibodies do not cross the placenta. This makes IgM antibody detection a reliable method for confirming congenital toxoplasmosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Detection of IgG antibodies indicates past exposure and does not confirm acute infection. IgG antibodies can cross the placenta, making it difficult to distinguish between maternal and fetal antibodies.
- **Option B:** While detecting a rising titer of antibodies can indicate recent infection, it is not as direct or as specific as detecting IgM antibodies for confirming congenital toxoplasmosis.
- **Option C:** This option might refer to other methods of diagnosis, such as PCR (polymerase chain reaction) for detecting *T. gondii* DNA, which is indeed a method used to confirm toxoplasmosis but is not a serologic test.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the presence of **IgM anti-Toxoplasma antibodies** in a newborn is diagnostic of congenital toxoplasmosis. This is because IgM antibodies do not cross the placenta, and their presence indicates that the fetus was producing antibodies in response to an infection.

## **Correct Answer:** D. IgM antibodies.