This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Obstetrics, Subject name: Gynaecology & Obstetrics
Which of these is true of fetal circulation? 
Select and explain. 
A. Fetal hea chambers work in series to effectively supply brain and hea with higher oxygenated blood, B. The ductus venosus is the main branch of umbilical aery, C. The oxygen content of blood delivered to hea from IVC is higher than that leaving the placenta, D. The more oxygenated blood in the right atrium crosses the foramen ovale to the left atrium
D: The more oxygenated blood in the right atrium crosses the foramen ovale to the left atrium
Answer: D: The more oxygenated blood in the right atrium crosses the foramen ovale to the left atrium 
Explanation: In the supply of blood from placenta to the pas of fetal body we see two systems (1) Placenta - umbilical vein - ductus venosus - medial half of IVC - R.A. - L.A. - L.V. - aoa - brain and hea (2) Placenta - umbilical vein - ductus venosus - lateral half of IVC (+blood from lower extremities) R.A. - R.V. - pulmonary trunk - ductus aeriosus - aoa - rest of the body Fetal hea chambers work in parallel and not in series, which effectively supplies brain and hea with more oxygenated blood . The ductus venosus is a branch of the umbilical vein (not umbilical aery). The single umbilical vein enters the fetus and divides ductus venosus and poal sinus. The ductus venosus is the main branch of the vein which traverses the liver and enters the IVC directly. The IVC also receives less oxygenated blood from lower body and thus the oxygen content of the blood delivered to the hea from IVC is less than that leaving the placenta through the umbilical vein. The 'more oxygenated' blood from R.A. crosses foramen ovale to L.A.