A 35-year-old woman from Assam presents with a 4-month history of fever, weight loss, and abdominal swelling. She received 28 days of sodium stibogluconate 3 months ago for kala-azar, with initial clinical improvement. Now she has relapsed with fever, hepatosplenomegaly, and a hemoglobin of 7.8 g/dL. Bone marrow smear again shows amastigotes. Which of the following is the most appropriate management?