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: Physiology
[{'content': 'S2 is due to: \nSelect and explain. \nA. Rapid gush of blood in ventricle, B. Atrial contraction, C. AV valve closure, D. Semilunar valve closure', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The Second Heart Sound Is Associated with Closure of the Aortic and Pulmonary Valves.\xa0\nThe second heart sound results from sudden closure of the semilunar valves (i.e., the aortic and pulmonary valves) at the end of systole.\xa0\nWhen\xa0 the\xa0 semilunar valves close, they bulge backward toward the ventricles and their elastic stretch recoils the blood back into the arteries, which causes a short period of\xa0 reverberation\xa0 of\xa0 blood\xa0 back\xa0 and\xa0 forth\xa0 between\xa0 the walls of the arteries and the semilunar valves, as well as between these valves and the ventricular walls. The vibrations occurring in the arterial walls are then transmitted mainly along the\xa0 arteries. When the vibrations of the vessels or ventricles come into contact with a “sounding board,” such as the chest wall, they create sound that can be heard.\n\xa0\nReference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION(INTERNATIONAL EDITION ) page no 282', 'role': 'assistant'}]