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 mEq/l,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:

• A. Alport's syndrome
• B. Medullary sponge kidney
• C. Chronic glomerulonephritis
• D. Nephronophthisis