A 12- year -old boy is referred for evaluation of nocturnal enuresis and sho stature. The blood pressure is normal. The blood urea is 112 mg/dl, creatinine 6 mg/d1, sodium 119 mEq/1, potassium 4 mEq/1, calcium 7mg/d1, phosphate 6mg/dl and alkaline phosphatase 400 U/l.Urinalysis shows trace proteinuria with hyaline casts; no red and white cells are seen. Ultrasound shows bilateral small kidneys and the micturating cystourethrogram is normal. The most likely diagnosis is: