A-13-year old boy is referred for evaluation of nocturnal enuresis and sho stature. His blood pressure is normal. The hemoglobin level is 8g/dl. urea 112 mg/d1, creatinine 6 mg/d1, sodium 119 mEq/ dl, potassium 4 mEq/1, calcium 7 mg/d1, phosphate 6 mg/d1 and alkaline phophatase 300 U/I. 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 ?