A 18-month-old child from rural Maharashtra presents with a 3-month history of poor weight gain, diarrhea, and skin changes. On examination, the child weighs 8 kg (expected 12 kg for age), has bilateral ankle edema, and sparse, discolored hair. Serum albumin is 1.8 g/dL, and total serum protein is 4.2 g/dL. What is the most appropriate immediate next step in management?