A 3-year-old boy has been hospitalized with severe edemas. Objectively, the boy is pale, his blood pressure is 90/60 mm Hg, he has oliguria. General urinalysis shows the following: color- yellow, specific gravity 1020, protein levels-3.5 g / L, erythrocytes leached,4-5 in the vision field, leukocytes 2-3 in the vision field, 24-hour proteinuria 6.6 g. The cholesterol levels are 9.8 mmol/L. What next step in the patient's management would be optimal?