**Core Concept**
Toxoplasmosis in the fetus is a congenital infection caused by the protozoan parasite *Toxoplasma gondii*, which can lead to severe complications if left untreated. This condition occurs when a pregnant woman acquires primary infection with *T. gondii* for the first time during pregnancy.

**Why the Correct Answer is Right**
The diagnosis of toxoplasmosis in the fetus is primarily based on serological tests in the mother and the fetus, as well as ultrasound findings. However, definitive confirmation of congenital toxoplasmosis requires the detection of *T. gondii* in the fetus's tissues, such as the placenta, amniotic fluid, or fetal blood. Polymerase chain reaction (PCR) is a sensitive method for detecting *T. gondii* DNA in these samples, allowing for early and accurate diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Serological tests in the mother can indicate past or recent infection, but they are not sufficient for confirming congenital toxoplasmosis, as they may not detect the parasite in the fetus.
**Option B:** Ultrasound findings can suggest fetal abnormalities associated with toxoplasmosis, but they do not directly confirm the presence of the parasite.
**Option C:** Blood tests in the fetus may show changes due to the infection, but they are not the most reliable method for confirming congenital toxoplasmosis.

**Clinical Pearl / High-Yield Fact**
Mnemonic: "TIGER" - Toxoplasmosis Infection during pregnancy can result in **T**issue damage, **I**nfertility, **G**enetic abnormalities, **E**ye problems, **R**etardation, and **S**erious birth defects.

**Correct Answer:** C.