**Core Concept**
Pyrimethamine and sulfadoxine are a combination of antifolate and sulfonamide drugs used in the treatment of malaria, particularly for Plasmodium vivax (P.vivax) infections. This combination works synergistically by inhibiting dihydrofolate reductase (DHFR) and dihydropteroate synthase enzymes, essential for parasite growth and survival.

**Why the Correct Answer is Right**
Pyrimethamine + sulfadoxine is a well-established and effective treatment for P.vivax malaria, especially in single-dose regimens. The combination is administered orally and can be given as a single dose due to its long-lasting effect against the malaria parasite. This makes it a convenient and effective option for treating P.vivax malaria in regions where resistance to other antimalarial drugs is a concern. The combination's mechanism of action involves the inhibition of dihydrofolate reductase (DHFR) and dihydropteroate synthase enzymes, which are crucial for the parasite's ability to synthesize folic acid and thus replicate.

**Why Each Wrong Option is Incorrect**
**Option A:** Atovaquone + Proguanil is a combination antimalarial used for treating P. falciparum malaria, not P.vivax. It is not suitable for a single-dose treatment regimen.
**Option B:** Artemisinin is a fast-acting antimalarial drug used primarily for treating severe malaria and P. falciparum infections. It is not typically used as a single-dose treatment for P.vivax malaria.
**Option C:** Quinine + Tetracycline is a combination used for treating P. falciparum malaria, but it is not the preferred regimen for P.vivax infections. Quinine is often used in combination with other antimalarial drugs for P.vivax treatment.

**Clinical Pearl / High-Yield Fact**
When treating P.vivax malaria, it's essential to note that the parasite may relapse from the liver stage, requiring a follow-up treatment with primaquine to eliminate the liver stages and prevent relapse.

**✓ Correct Answer: D. Pyrimethamine + Sulfadoxime**