A 32-year-old woman from rural Odisha presents with fever, chills, and headache for 5 days. She had returned from a malaria-endemic area 10 days ago. Blood smear shows P. vivax parasitemia (2% RBCs infected). She is started on chloroquine 600 mg base stat, then 300 mg at 6 hours, 24 hours, and 48 hours. On day 7 of treatment, she develops recurrent fever with parasitemia on repeat blood smear. Which additional drug is most appropriate to prevent relapse?