The pregnancy of a 28-year-old woman with one previous miscarriage is complicated by idiopathic polyhydramnios, which was first recognized by routine ultrasound at the end of the second trimester. After therapeutic amniocentesis (estimated amniotic fluid volume of 10 l) and rupture of membranes at 30 weeks of gestation, an acute Caesarean section was performed. The delivered male infant was appropriate for gestational age and showed an uneventful postnatal adaptation, except for a mild respiratory distress syndrome requiring a positive end-expiratory pressure device. However, a few days later, he developed hyponatremia, hyperkalemia, hyposthenuria, hypercalciuria, and a weight loss from birthweight by >15%. The diagnosis most likely is: