A 28-year-old man from endemic Assam presents with fever, headache, and myalgia for 4 days. Thick and thin blood smears confirm Plasmodium vivax malaria. He is started on chloroquine 600 mg base stat, then 300 mg at 6 hours, then 300 mg daily for 2 days, followed by primaquine 15 mg base daily for 14 days to prevent relapse. On day 5 of primaquine therapy, he develops dark urine, jaundice, and pallor. Hemoglobin drops from 13.5 g/dL to 10.2 g/dL. Reticulocyte count is elevated at 8%. What is the most likely diagnosis and the immediate next step?