Hi, Thanks for your question. Hydronephrosis of kidney is due to obstruction to the flow of urine. When unilateral the obstruction is on the same side at the level of ureter (tube connecting kidney to urine bag) or vesicoureteric junction (junction of ureter with urine bag) . When hydronephrosis is bilateral the obstruction is at the level of urine bag (urinary bladder) or urethra. As your niece has right side hydronephrosis, there may be obstruction of right side ureter or vesicoureteric junction. Rarely obstruction at the level of urinary bladder and urethra may give rise to unilateral hydronephrosis initially. Her renal isotope scan confirm the presence of hydronephrosis. Renal isotope scan help in assessing kidney function. MCU (Micturating Cysto-Urethrogram) help in establishing site of obstruction. Considering her age and female sex there is high possibility of vesicoureteric reflux, it means there is back blow of urine from urinary bladder to ureter. Depending upon its grade appropriate treatment can be planned. There may be need for surgical correction in some cases depending upon grade. You should consult urosurgeon or pediatric surgeon for appropriate treatment. Hope this helps you, if so do vote as helpful.