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This patient who presents with a continuous polyphasic murmur that was not present at birth most likely has a patent ductus arteriosus. This structure is derived from the embryonic 6th branchial arch. The ductus arteriosus is an embryological connection between the pulmonary artery and the aorta that allows the fetal circulation to bypass the immature lungs. It is derived from the 6th aortic arch as part of the 6th branchial arch. This connection normally closes in the immediate postpartum period and can cause left to right shunting of blood when it fails to close. In the neonatal period, large defects can present as respiratory distress and poor feeding. Over time, the increased blood flow through the pulmonary vasculature can result in pulmonary hypertension and reversal of the shunt. The classic physical exam finding associated with this disease is a continuous polyphasic "machine-like" murmur that is not present at birth but occurs as the ductus begins to close. Incorrect Answers: Answer 1: Branchial arch 1 derivatives include the maxillary artery and the external carotid artery. Failure of arch development leads to Treacher-Collins syndrome, which presents with midface hypoplasia and malformed ears. Answer 2: Branchial arch 2 derivatives include the stapedial artery and the hyoid artery. An abnormal connection between the cleft and pouch at this level can result in a congenital pharyngocutaneous fistula near the anterior border of the sternocleidomastoid. Answer 3: Branchial arch 3 derivatives include the common carotid arteries and the proximal part of the internal carotid arteries. No commonly observed abnormalities occur in this section of the vasculature as the variants are likely lethal in the embryonic state. Answer 4: Branchial arch 5 has no normal post-embryonic derivatives because it degenerates during development prior to birth. While abnormal persistence of this arch can result in aberrant connections, a far more common cause of left to right shunting is the persistence of a patent ductus arteriosus. Bullet Summary: The ductus arteriosus is an embryologic connection between the pulmonary artery and the aorta that is derived from the 6th aortic arch as part of the 6th branchial arch.
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