A 2-day-old male neonate born at term to a 28-year-old primigravida presents with persistent cyanosis (SpO₂ 78% on room air) and mild respiratory distress. On examination, the infant has a single loud S₂ and a systolic ejection murmur at the left upper sternal border. Chest X-ray shows an 'egg-on-string' appearance with pulmonary plethora. The ductus venosus has already closed. Which fetal vascular structure is most likely still patent and responsible for the right-to-left shunt in this neonate?