**Core Concept**
Toxoplasmosis is a parasitic infection caused by Toxoplasma gondii, which can be transmitted to humans through contaminated food or water, or congenitally from mother to fetus during pregnancy. In the first trimester of pregnancy, the risk of congenital toxoplasmosis is highest, and therefore, early and effective treatment is crucial.

**Why the Correct Answer is Right**
The drug of choice for treating toxoplasmosis in the first trimester of pregnancy is spiramycin, a macrolide antibiotic that has been shown to be effective in reducing the transmission of Toxoplasma gondii to the fetus. Spiramycin works by inhibiting protein synthesis in the parasite, ultimately leading to its death. This is particularly important in the first trimester, when the risk of congenital toxoplasmosis is highest.

**Why Each Wrong Option is Incorrect**
**Option A:** Pyrimethamine is an antiparasitic drug, but it is not the drug of choice for treating toxoplasmosis in pregnancy due to its potential teratogenic effects.
**Option B:** Folinic acid is used to prevent the hematologic toxicity associated with pyrimethamine, but it is not used to treat toxoplasmosis itself.
**Option C:** Sulfadiazine is an antibacterial drug that is sometimes used in combination with pyrimethamine to treat toxoplasmosis, but it is not the preferred treatment for pregnant women due to its potential risks.

**Clinical Pearl / High-Yield Fact**
Remember that spiramycin is the drug of choice for treating toxoplasmosis in pregnancy, and it should be initiated as soon as possible after diagnosis to minimize the risk of congenital transmission.

**Correct Answer:** A. Spiramycin.