A 1-mоnth-оld girl presents with fever аnd vоmiting. Her serum WBC count is elevаted. UA reveаls 100 WBC per high power field (unspun). It is positive for nitrates and leukocyte esterase. Urine culture results confirm a UTI. Renal USG and VCUG show mild to moderate hydronephrosis and grade III VUR on the right. Which of the following is the best next step?