AN 18-MONTH-OLD girl was noted to be pale but in no distress (hemogloblin, 6.9 g/L) and below the fifth percentile for weight (8.27 kg) during a routine follow-up visit. Iron supplementation did not change her hemoglobin concentration. Her serum creatinine level was 123 μmol/L (1.4 mg/dL) and her serum urea nitrogen level was 23 mmol/L (64 mg/dL). A renal ultrasound scan showed mild pyelocaliectasis; a voiding cystourethrogram was normal. Her urine output ranged from 2 to 2.5 L/d.
Her medical history was remarkable for perinatal complications including seizures, cardiac arrest, meconium ileus, and pneumonia. Her postnatal hospital stay was 9 days. She was hospitalized at age 6 months for pneumonia. Her only other notable medical history was chronic constipation. Her family history was negative for renal or urologic diseases. Her medications on presentation were a stool softener and an iron supplement.
On physical examination her weight was 8.27 kg (below