A 32-year-old woman from Odisha presents with fever, chills, and sweating for 3 days. Blood smear shows P. vivax trophozoites. She is started on chloroquine 600 mg base stat, followed by 300 mg at 6, 24, and 48 hours. On day 5 of treatment, fever recurs with similar parasitaemia. What is the most appropriate next step in management?