USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What cardiac condition involves a continuous machine-like murmur indicative of a left-to-right shunt?

1. Coarctation of the aorta

2. Patent ductus arteriosus

3. Aortic stenosis

4. Mitral regurgitation

The correct answer is: Patent ductus arteriosus

The condition characterized by a continuous machine-like murmur indicative of a left-to-right shunt is indeed patent ductus arteriosus (PDA). This condition occurs when the ductus arteriosus, a normal fetal blood vessel that connects the pulmonary artery to the aorta, fails to close after birth. As a result, blood flows from the aorta (higher pressure) to the pulmonary artery (lower pressure), causing a left-to-right shunt. This continuous flow of blood creates the distinctive machine-like murmur, which can be heard throughout the cardiac cycle, as opposed to murmurs that are typically heard only during specific phases, such as systole or diastole. Other conditions mentioned have different mechanisms and presentations. For example, coarctation of the aorta may lead to hypertension and different types of murmurs due to differential pressure across the aorta, but does not produce the continuous murmur characteristic of PDA. Aortic stenosis typically presents with a systolic ejection murmur and does not involve a shunt, while mitral regurgitation results in a holosystolic murmur due to backflow from the left ventricle into the left atrium but is also not associated with a left