A 16-year-old male presents with a history of recurrent urinary tract infections and vague lower abdominal discomfort. On physical examination, there is mild tenderness in the lower abdomen, but no palpable masses are appreciated. A renal ultrasound is obtained and reveals that the lower poles of the kidneys appear fused across the midline, forming a U-shaped structure anterior to the great vessels. Which of the following congenital renal anomalies is most consistent with these imaging findings?