USMLE Step 1 Pathology Practice Test

The continuous "machinery" murmur is classically associated with patent ductus arteriosus (PDA). This condition occurs when the ductus arteriosus, a fetal blood vessel that connects the pulmonary artery to the aorta, fails to close after birth. As a result, blood flows continuously from the aorta into the pulmonary artery, creating a characteristic sound. This murmur is heard throughout the cardiac cycle, unlike typical systolic or diastolic murmurs, and has a distinctive "machinery-like" quality, which is often described as being similar to the sound of a steam engine.

The presence of the continuous murmur in PDA is due to the different pressures in the aorta and pulmonary artery throughout the heart cycle, allowing for blood flow from the high-pressure aorta to the lower-pressure pulmonary artery. This hallmark feature of PDA differentiates it from other heart conditions that may present with murmurs, which typically have a more localized or time-limited character in relation to the phases of the cardiac cycle.