A 2-year-old girl from urban Delhi presents with a 3-week history of progressive swelling of face, limbs, and abdomen. She has been eating a diet predominantly of rice and lentils with minimal animal protein. On examination, she is lethargic, has bilateral pitting edema, hepatomegaly (3 cm below costal margin), and a characteristic 'flaky paint' dermatitis on the extensor surfaces. Serum albumin is 1.8 g/dL, and she has hypoglycemia (blood glucose 45 mg/dL). What is the most appropriate immediate next step?