Tetralogy of Fallot and transposition of great aeries are cyanotic hea diseases and rheumatic fever will not occur in this age group. So the answer is ventricular septal defect. When the effective area of the defect is large, there is viually no resistance to flow, and systemic systolic pressure is present in the right ventricle and the pulmonary aery. At bih, pulmonary vascular resistance is high, with little if any left-to-right shunt, despite the presence of a large defect. This resistance gradually falls over the first few weeks of life, permitting a progressively greater amount of blood to flow through the defect, to the pulmonary aeries and back to the left atrium and left ventricle. In many infants, the left ventricular volume overload eventually leads to left ventricular "failure," with elevated left ventricular end-diastolic and left atrial pressures and pulmonary congestion. With a large VSD and a high pulmonary vascular resistance, the systolic murmur becomes soft and sho because there is little left-to-right shunt across a large defect, and a right-to-left shunt across the VSD causes little or no murmur. Ref: Brown D.W., Fulton D.R. (2011). Chapter 83. Congenital Hea Disease in Children and Adolescents. In V. Fuster, R.A. Walsh, R.A. Harrington (Eds), Hurst's The Hea, 13e.