Renal Calculi (Ureteric Stone)

A 45-year-old man presents to the emergency department with sudden-onset severe right flank pain radiating to the groin and testis, associated with nausea and vomiting. Urinalysis shows microscopic hematuria without pyuria. Renal ultrasound demonstrates a 7 mm hyperechoic focus at the right ureterovesical junction with posterior acoustic shadowing, and on color Doppler imaging a characteristic **rapidly alternating color signal behind the calculus** (marked **C** in the diagram). The right kidney shows grade 2 hydronephrosis. Which of the following best describes the significance of the finding marked **C** in confirming the diagnosis of nephrolithiasis?