**Core Concept:**
Malaria is a life-threatening disease caused by protozoan parasites of the Plasmodium genus, primarily affecting the red blood cells. Plasmodium vivax is one such parasite responsible for causing malaria in humans. In pregnant women, malaria can lead to severe anemia, hypoglycemia, and even maternal and fetal complications. Treatment options need to address the patient's specific condition and minimize potential harm to the fetus.

**Why the Correct Answer is Right:**
The correct answer, chloroquine, is an effective antimalarial drug with minimal fetal toxicity. Chloroquine is a 4-aminoquinoline drug that inhibits the parasite's ability to produce its protective coat (hemozoin) within the infected red blood cells. Its minimal fetal toxicity is due to its preferential accumulation in red blood cells over placental cells, reducing the risk to the fetus.

**Why Each Wrong Option is Incorrect:**
A. Artemisinin derivatives like dihydroartemisinin or artesunate are not the first-line treatment for pregnant women as they can cause fetal toxicity and be associated with congenital malaria.
B. Mefloquine is a second-line antimalarial drug with increased fetal toxicity risk.
C. Pyrimethamine is an antifolate drug primarily used for treating multidrug-resistant Plasmodium falciparum malaria, not P. vivax infections in pregnant women.

**Clinical Pearl:**
When treating malaria in pregnant women, it is essential to choose drugs that are safe for the fetus, such as chloroquine, while ensuring adequate parasite clearance to prevent severe complications. In case of severe or complicated malaria, therapy should be combined with additional drugs like sulphadoxine-pyrimethamine or quinine (not recommended for first trimester) to address the complexity of treating malaria in pregnancy.