Correct Answer: C. Right 4th aortic arch
The right 4th aortic arch is the embryological origin of the right subclavian artery. Abnormal development (persistence or hypoplasia) of the right 4th arch results in an anomalous right subclavian artery (ARSA), which arises from the descending thoracic aorta distal to the left subclavian artery and crosses the midline posterior to the esophagus to reach the right arm. This is the most common aortic arch anomaly in the Indian population, occurring in ~0.5–2% of autopsies. While often asymptomatic, ARSA can cause dysphagia lusoria (difficulty swallowing) if the artery compresses the esophagus, or stridor if it compresses the trachea—particularly when the artery is accompanied by a diverticulum of Kommerell. The embryological basis is that the right 4th arch normally forms the right subclavian artery; when it regresses, the subclavian artery must originate from the aorta distal to its normal origin, creating the characteristic retroesophageal course. This is the defect most commonly imaged on chest CT and echocardiography in Indian cardiac centers.
Why the other options are wrong
A. Left 4th aortic arch — The left 4th aortic arch forms the normal left subclavian artery and is retained in all normal individuals. Abnormality of the left 4th arch would not produce the classic anomalous right subclavian artery pattern shown. This option confuses laterality—the defect involves the RIGHT side, not the left. B. Left 6th aortic arch — The left 6th arch forms the left pulmonary artery and ductus arteriosus. Abnormality here causes patent ductus arteriosus (PDA) or pulmonary artery anomalies, not anomalous subclavian artery. This is a common NBE trap—6th arch anomalies are frequent, but they do not produce retroesophageal subclavian artery. D. Right 6th aortic arch — The right 6th arch normally regresses completely and does not contribute to normal adult anatomy. Abnormality of the right 6th arch is embryologically irrelevant to subclavian artery formation. The 6th arch is involved in pulmonary circulation, not systemic arch derivatives.
High-Yield Facts
- Anomalous right subclavian artery (ARSA) arises from the descending thoracic aorta distal to the left subclavian and crosses posterior to the esophagus—most common aortic arch anomaly.
- Right 4th aortic arch normally forms the right subclavian artery; regression causes ARSA with retroesophageal course.
- Dysphagia lusoria (difficulty swallowing) occurs when ARSA or its diverticulum of Kommerell compresses the esophagus—classic clinical presentation.
- Left 4th arch forms the normal left subclavian artery and is always retained; left 6th arch forms left pulmonary artery and ductus arteriosus.
- ARSA is often asymptomatic but may present with stridor, dysphagia, or chest pain; diagnosis confirmed on CT angiography or echocardiography.
Mnemonics
4th Arch = Subclavian; 6th Arch = Pulmonary 4th arch → Subclavian artery (right 4th = right subclavian). 6th arch → Pulmonary artery + ductus. When right 4th regresses → ARSA (retroesophageal). When 6th abnormal → PDA or pulmonary anomaly. ARSA = Anomalous Right Subclavian Artery Right 4th arch defect → ARSA → crosses behind esophagus → dysphagia lusoria. Remember: Anomalous Right Subclavian = Arch Retroesophageal Swallow problem.
NBE Trap
NBE pairs "6th aortic arch" with vascular anomalies to lure students who recall that 6th arch anomalies are common (PDA, pulmonary artery sling). However, the 6th arch does NOT form the subclavian artery—only the 4th arch does. The trap is confusing frequency of 6th arch anomalies with the embryology of subclavian artery origin.
Clinical Pearl
In Indian cardiac centers, ARSA is often discovered incidentally on CT chest done for other indications (pneumonia, TB screening). A patient presenting with dysphagia and a retroesophageal vascular structure on imaging should immediately raise suspicion for ARSA—this is a classic bedside diagnosis that prevents unnecessary endoscopy.
_Reference: Robbins Ch. 12 (Congenital Heart Disease); Harrison Ch. 282 (Aortic Diseases); Langman's Embryology (Aortic Arch Development)_