**Core Concept**
Toxoplasmosis is a parasitic infection caused by Toxoplasma gondii, which can be transmitted from mother to fetus during pregnancy, leading to congenital toxoplasmosis. Preventing this transmission is crucial, and antiparasitic medications play a vital role.

**Why the Correct Answer is Right**
Spiramycin is a macrolide antibiotic that has been shown to reduce the transmission of Toxoplasma gondii from mother to fetus. It works by inhibiting protein synthesis in the parasite, ultimately reducing the parasite load in the placenta. Spiramycin is the preferred treatment for pregnant women with acute toxoplasmosis, as it is effective in preventing congenital transmission without causing significant harm to the fetus. This is in contrast to other options, which may have more severe side effects or are not recommended during pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Pyrimethamine is an antiparasitic medication, but it is not the preferred choice for preventing fetal toxoplasmosis during pregnancy. Pyrimethamine can cause significant fetal abnormalities and is generally avoided in pregnant women.

**Option B:** Sulfadiazine is an antibiotic that is sometimes used in combination with pyrimethamine to treat toxoplasmosis. However, it is not the preferred choice for preventing fetal toxoplasmosis during pregnancy due to the risk of kernicterus in the newborn.

**Option D:** Pyrimethamine + Sulfadiazine is a combination therapy that is sometimes used to treat toxoplasmosis in immunocompromised patients. However, it is not the preferred choice for preventing fetal toxoplasmosis during pregnancy due to the risk of fetal abnormalities and kernicterus.

**Clinical Pearl / High-Yield Fact**
It's essential for pregnant women to avoid undercooked meat, unwashed vegetables, and cat feces to reduce the risk of toxoplasmosis transmission. Spiramycin can be effective in preventing congenital toxoplasmosis, but it's crucial to diagnose and treat the infection promptly to minimize the risk of transmission.

**✓ Correct Answer: C. Spiramycin**