The child in the question likely has renal vein thrombosis, a condition more commonly seen in infants who are dehydrated, had birth depression, have polycythemia or were born to diabetic mothers. The child in the case was large for gestational age, a hint that gestational diabetes might have been present and which also can result in polycythemia. Children with renal vein thrombosis can present with history of oliguria and hematuria, and on examination enlarged kidneys may be palpable. Confirmation of the diagnosis is by renal ultrasound with renal vein Doppler studies. UTI in a 3-day- old infant is unusual, and the presentation with abdominal mass without temperature instability is unlikely making a urine catheterization and intravenous antibiotics a lower priority. Abdominal tumors such as Wilms or neuroblastoma are less likely in such a young infant making CT scans or urine catecholamine measurements a lesser priority.