A 47-year-old man comes to the emergency department with acute left flank pain radiating to the groin and nausea. He has had similar episodes in the past. Urinalysis shows numerous red blood cells but no casts. Noncontrast CT demonstrates a 6-mm calculus at the ureteropelvic junction with dilation of the collecting system proximal to the obstruction.

Which of the following structures is the dilated, funnel-shaped proximal portion of the ureter that directly receives urine from the major and minor calyces?