Final answer:
A mid systolic murmur at the left upper sternal border with right atrial and ventricular dilation suggests a condition like pulmonary valve stenosis or an atrial septal defect.
Step-by-step explanation:
A mid systolic murmur at the left upper sternal border with right atrial and ventricular dilation is most suggestive of an underlying condition affecting the heart, potentially a valvular heart disease such as pulmonary valve stenosis or an atrial septal defect causing a left-to-right shunt.
These conditions can create turbulent blood flow through the heart, leading to the auscultation of a murmur with a stethoscope. During systole, which is the contraction phase of the cardiac cycle when the ventricles pump blood into the arteries, heart sounds known as "lub" and "dub" are heard due to the closing of the atrioventricular valves.
If these valves or the surrounding cardiac structure are abnormal, such as in hypertrophic cardiomyopathy, which is a pathological enlargement of the heart generally for no known reason, it can manifest as a murmur. The described symptoms hint at a condition that has led to the dilation of the right heart chambers.