This was an honors question. You should order an ultrasound. More than 75% of all abdominal masses in children are caused by abnormalities in the urinary tract. Of those, the most common masses, by far, are cystic (hydronephrosis). An ultrasound will show you whether the mass is cystic or solid. If a cystic mass is seen in the urinary tract, you should assess whether the mass was caused by obstruction of the kidney or ureter using a nuclear renal scan or an intravenous pyelogram (IVP). In addition, an ultrasound is non-invasive and much less expensive than a CAT scan.
Notice that the kidney is distorted. The central echoes that are normally present in the hilum are replaced by a large heterogeneous mass replacing the lower pole of the kidney. In this case, the mass was solid. At that point you should order a CAT scan.
