A 35-year-old man with a history of recurrent nephrolithiasis presents with acute flank pain, hematuria, and a rise in serum creatinine from 0.9 to 2.1 mg/dL over 6 hours. Urine output is 150 mL/day. Bedside ultrasound shows bilateral hydronephrosis. Which investigation is most appropriate to confirm the diagnosis and guide management?