A 28-year-old woman from Jharkhand presents with fever, headache, and vomiting for 2 days. Thick blood smear shows P. falciparum parasitaemia (4% of RBCs). She is alert, oriented, with no focal neurological signs. Temperature is 38.9°C, BP 110/70 mmHg, RR 22/min, and oxygen saturation 96% on room air. Serum creatinine is 1.2 mg/dL (baseline 0.8), and lactate is 3.2 mmol/L. What is the most appropriate immediate next step?