An 8-year-old child suffering from recurrent attacks of polyurea since childhood presents to the paediatrics OPD. On examination, the child is short statured vitals and BP are normal. Serum Creatinine - 6 mg%,HCO3 - 16 meq, Na - 134, K+ 4.2 . On USG, bilateral small kidneys are seen. Diagnosis is -