A 58-year-old woman presents with a 3-day history of right upper quadrant pain radiating to the back, jaundice (total bilirubin 6.8 mg/dL), pale stools, and dark urine. Liver enzymes show an obstructive pattern with ALP 480 U/L and GGT 320 U/L. Transabdominal ultrasound of the biliary tree is performed. The structure marked **A** in the diagram shows a curvilinear echogenic focus approximately 8 mm long with sharp posterior acoustic shadowing within the distal common bile duct. Which of the following best describes the diagnostic significance of this sonographic finding?