**Core Concept:**
Malaria is a vector-borne disease caused by Plasmodium parasites, with P. vivax being one of the species that can infect humans. During pregnancy, malaria can lead to severe maternal and fetal complications, emphasizing the importance of prompt and appropriate treatment.

**Why the Correct Answer is Right:**
The correct answer is Doxycycline. Doxycycline is a tetracycline antibiotic with excellent safety profile during pregnancy, as it crosses the placenta and has no known teratogenic effects. It has demonstrated efficacy against P. vivax malaria, making it the drug of choice for treating P. vivax malaria in pregnant women.

**Why Each Wrong Option is Incorrect:**
A. Artemisinin-combination therapy (ACT) options like Artemether-Lumefantrine (A), Artesunate-Amodiaquine (B), and Artemisinin-based combination therapy (ACTs) in general are inappropriate due to concerns about potential teratogenicity and other side effects.
C. Quinine is a first-line treatment for uncomplicated malaria but is not recommended for pregnant women due to potential side effects and contraindications.
D. Mefloquine is not suitable for pregnant women due to concerns about potential teratogenicity and other side effects.

**Clinical Pearl:**
Doxycycline is a safe and effective treatment option for P. vivax malaria in pregnant women, emphasizing the importance of considering drug safety profiles during medical decision-making in this vulnerable population. Instruct patients about potential side effects and monitor them accordingly.

**Correct Answer:** Doxycycline