**Core Concept**
Radical cure of Plasmodium vivax involves the elimination of the dormant liver stage of the parasite, known as hypnozoites, which can cause relapses. This requires a combination of antimalarial drugs that target both the blood stage and the liver stage of the parasite.

**Why the Correct Answer is Right**
The correct answer is chloroquine combined with primaquine. Chloroquine is effective against the blood stage of the parasite, while primaquine targets the liver stage by inducing the production of reactive oxygen species that destroy the hypnozoites. This combination is essential for a radical cure of P. vivax malaria.

**Why Each Wrong Option is Incorrect**
**Option A:** Chloroquine alone is not sufficient for a radical cure of P. vivax, as it does not target the liver stage.
**Option B:** Artemisinin-based combination therapies (ACTs) are effective against the blood stage of P. falciparum, but not against the liver stage of P. vivax.
**Option C:** Doxycycline is an antibiotic that has antimalarial activity, but it is not used for the radical cure of P. vivax.

**Clinical Pearl / High-Yield Fact**
Primaquine is contraindicated in individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency, as it can cause hemolysis. Always check for G6PD deficiency before prescribing primaquine.

**Correct Answer: D. Chloroquine combined with primaquine.**