A 3-month-old boy was seen for routine follow-up at the pediatric nephrology outpatient clinic. He had been diagnosed as having Sotos syndrome manifesting with craniofacial dysmorphism, feeding difficulties, pulmonary artery stenosis, and atrial septal defect, as well as complex urological abnormalities. He had bilateral hydronephrosis with megaureter and grade V vesicoureteral reflux to the left and grade I to the right kidney. At the age of 6 weeks, static renal scintigraphy using DMSA ( 99m Tc-dimercaptosuccinic acid) to assess renal morphology, structure, and function had demonstrated almost symmetrical kidney function (split kidney function left 44% vs right 56%) without cortical scarring.While his baseline serum creatinine had been 40 mol/L (0.45 mg/dL), a sudden rise to 69 mol/L (0.79 mg/dL) was noted. Urinary tract infection was ruled out, as was dehydration. On renal ultrasound, dilation of the right collecting system and ureter had increased significantly and a novel fluid collection at the upper pole was noted, which prompted an MRI study (Fig.