Embryology USMLE Step 1 Practice Question
A 5-day-old male neonate presents with severe cyanosis, poor feeding, and respiratory distress. Vital signs show HR 168 bpm, RR 52/min, BP 52/28 mmHg, SpO2 68% on room air, and temperature 37.2°C. Chest X-ray reveals an "egg-on-string" appearance. Echocardiography confirms transposition of the great arteries with patent foramen ovale allowing right-to-left shunting. The infant has no murmur on auscultation. Which embryologic defect of the conotruncus accounts for this reversal of the aorta and pulmonary artery origins?
Answer choices
- AFailure of neural crest cells to migrate into the conus cordisCorrect answer
- BIncomplete fusion of the dorsal mesocardium
- CInsufficient endocardial cushion tissue formation
- DAbnormal development of the sinus venosus
- EFailure of the common arterial trunk to septate
- FAbnormal rotation of the embryonic cardiac loop during looping morphogenesis
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