<prompt> The fourth heart sound is caused by:Closure of the aortic and pulmonary valvesVibrations in the ventricular wall during systoleVentricular fillingClosure of the mitral and tricuspid valves  <response>: Atrial Contraction Sound Fourth Heart Sound. An atrial heart sound can sometimes be recorded in the phonocardiogram, but it can almost never be heard with a stethoscope because of its weakness and very low frequencyusually 20 cyclessec or less. This sound occurs when the atria contract, and presumably, it is caused by the inrush of blood into the ventricles, which initiates vibrations similar to those of the third heart sound. A fourth heart sound is common in persons who derive benefit from atrial contraction for ventricular 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.Reference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITIONINTERNATIONAL EDITION page no 284 