A 12-year-old boy is referred for evaluation of nocturnal enuresis and short stature. The blood pressure is normal. The blood urea is 112 mg/dl, creatinine 6 mg/dl, sodium 119 mEq/l, potassium 4 mEcill, calcium 7 mg/dl, phosphate 6 mg/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 –