Gross — Aortic Coarctation Postductal Indentation

A 7-year-old boy presents to the outpatient clinic with complaints of headaches and leg fatigue on exertion. On examination, his blood pressure in the right arm is 145/92 mmHg, while in the right leg it is 98/60 mmHg. Femoral pulses are diminished bilaterally, and a harsh systolic murmur is heard over the left infraclavicular region. Chest X-ray shows a "figure-of-3 sign" with rib notching of the inferior borders of ribs 3–8. The structure marked **A** in the diagram represents the site of pathology. Which of the following best describes the anatomic location and hemodynamic consequence of the lesion marked **A**?