An infant of a mother with type I. Diabetes has a PCV of 68%. On day 3 of life, he suddenly manifests gross hematuria and a flank mass on the left. Most likely diagnosis is:

Renal venous thrombosis in the newborn period commonly presents with gross hematoria and a flank mass. Infants of diabetic mothers by viue of polycythemia are at slightly increased risk for this condition. A meconium plug may cause abdominal distention, but probably would not cause a discrete flank mass and would not lead to gross hematuria. A neuroblastoma would not produce hematuria nor grow so fast as to suddenly produce a flank mass. A ureteropelvic junction obstruction is usually not associated with hematuria and should have been palpable at the time of bih. An adrenal hemorrhage may present as a sizeable mass but would not be associated with gross hematuria.