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**?

Explanation

## Why option 1 is correct The structure marked **A** represents the postductal (adult-type) aortic coarctation — a discrete shelf-like narrowing located just distal to the left subclavian artery at the site of the ligamentum arteriosum. This is the most common form in older children and adults (Robbins 10e, Ch 12). The narrowing proximal to the coarctation causes upper extremity hypertension (145/92 mmHg in the arm), while the obstruction to flow results in lower extremity hypotension (98/60 mmHg in the leg). The clinical findings in this case — headache, leg fatigue, diminished femoral pulses, radio-femoral delay, harsh systolic murmur over the left infraclavicular region, and rib notching from intercostal collateral arteries — are all hallmark features of postductal coarctation. The "figure-of-3 sign" on CXR reflects pre-stenotic dilation, the narrow coarctation segment, and post-stenotic dilation. ## Why each distractor is wrong - **Option 2**: This describes preductal (infantile-type) coarctation, which occurs proximal to the ductus arteriosus and presents in infancy with ductal-dependent circulation and differential cyanosis (pink upper body, blue lower body). The patient's age (7 years) and clinical presentation are inconsistent with preductal coarctation. - **Option 3**: While bicuspid aortic valve is the most common association with aortic coarctation (50–85%), it is not the primary lesion marked **A**. The coarctation itself is the discrete aortic narrowing, not the aortic root or valve dysfunction. - **Option 4**: The coarctation is located in the descending thoracic aorta at the level of the ligamentum arteriosum (just distal to the left subclavian), not at the diaphragm. Mesenteric ischemia is not a typical presentation of postductal coarctation. **High-Yield:** Postductal coarctation = discrete narrowing just distal to left subclavian at ligamentum arteriosum; hallmark finding is upper extremity hypertension with lower extremity hypotension and radio-femoral delay. [cite: Robbins and Cotran Pathologic Basis of Disease, 10th edition, Chapter 12]