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
Which of the following structures form the bulge marked X? 
Select and explain. 
A. Anterior Aortic sinus, B. Right posterior Aortic sinus, C. Left posterior Aortic sinus, D. pulmonary trunk
B: Right posterior Aortic sinus
Answer: B: Right posterior Aortic sinus 
Explanation: The X marked bulge in Right atrium is Torus Aorticus formed by Right posterior Aortic sinus.