This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
[{'content': 'Which of the following structures form the bulge marked X? \nSelect and explain. \nA. Anterior Aortic sinus, B. Right posterior Aortic sinus, C. Left posterior Aortic sinus, D. pulmonary trunk', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The X marked bulge in Right atrium is Torus Aorticus formed by Right posterior Aortic sinus.', 'role': 'assistant'}]