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': 'The fourth heart sound is caused by: \nSelect and explain. \nA. Closure of the aortic and pulmonary valves, B. Vibrations  in  the  ventricular  wall during systole, C. Ventricular filling, D. Closure  of  the  mitral  and  tricuspid valves', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Atrial Contraction Sound (Fourth Heart Sound).\xa0\nAn\xa0 atrial\xa0 heart\xa0 sound\xa0 can\xa0 sometimes\xa0 be\xa0 recorded\xa0 in the phonocardiogram, but it can almost never be heard with a stethoscope because of its weakness and very low frequency—usually\xa0 20\xa0 cycles/sec\xa0 or\xa0 less.\xa0 This\xa0 sound occurs\xa0 when\xa0 the\xa0 atria\xa0 contract,\xa0 and\xa0 presumably,\xa0 it\xa0 is caused by the inrush of blood into the ventricles, which initiates\xa0vibrations\xa0 similar\xa0to\xa0those\xa0of\xa0the\xa0third\xa0heart sound. A fourth heart sound is common in persons who derive\xa0 benefit\xa0from\xa0atrial\xa0contraction\xa0for\xa0ventricular filling as a result of decreased ventricular wall compliance and increased resistance to ventricular filling. For example, a fourth heart sound is often heard in older patients with left ventricular hypertrophy.\nReference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION(INTERNATIONAL EDITION ) page no 284', 'role': 'assistant'}]