A 4-hour-old term male infant is being examined by a neonatologist. The mother is 26 years old gravida 1, para 1 and has no history of vaccination. She reports a few episodes of fever during the first and second trimester. Delivery was uneventful, and the infant was noted to have low birth weight. On physical examination, he is found to have absent red reflex in both eyes, a small head, and a continuous murmur over the left sternal border.
Which of the following is the embryological origin of the structure causing the murmur?
D: The 6th branchial arch
Fever during pregnancy can have many etiologies. In unvaccinated mothers, rubella should be considered. This infant boy shows signs of congenital rubella syndrome. Clinical manifestations include hearing loss, heart defects, microcephaly, cataracts, low birth weight, and intellectual disability. One of the potential heart defects is patent ductus arteriosus, which presents with a continuous “machine-like” murmur heard best at the left sternal border. The ductus arteriosus originates from the 6th branchial arch which also forms the pulmonary trunks.
Answer choice A: Allantoic duct, is incorrect. This duct connects the dome of the bladder to the yolk sac during pregnancy. It regresses into the urachus.
Answer choice B: Bulbus cordis, is incorrect: The bulbus cordis elongates and forms three parts: the trabeculated portion of the right ventricle, the conus cordis which is the outflow tract, and the asending aorta and pulmonary trunk.
Answer choice C: The 5th branchial arch, is incorrect. The 5th branchial arch involutes during development and does not contribute to the formation of adult structures.
Answer choice E: Truncus arteriosus, is incorrect. This structure forms the aorta and the pulmonary trunk.
Key Learning Point
Unvaccinated mothers have an increased fetal risk of congenital rubella syndrome which can present with bilateral cataracts, sensorineural hearing loss and congenital heart disease, which includes patent ductus arteriosus. The ductus arteriosus derives from the 6th aortic arch. During embryonic development, this connection forms between the first portion of the pulmonary artery and the isthmus of the aorta. This provides a path through which fetal blood can be shunted away from the pulmonary circuit and delivered directly into the systemic circulation. After birth, the ductus arteriosus closes, and its remnant is referred to as the ligamentum arteriosum. A failure of the closure of the ductus arteriosus leads to a patent ductus arteriosus (PDA) which may be treated medically using non-steroidal anti-inflammatory agents (NSAIDs) or with surgical techniques.
