What is the most likely diagnosis for a child with a continuous machinery murmur?

A continuous machinery murmur is primarily associated with patent ductus arteriosus (PDA), which is a condition where the ductus arteriosus fails to close after birth. This structure is a normal fetal blood vessel that allows blood to bypass the lungs while in utero. After birth, if it remains open, it creates a path for blood to flow from the aorta to the pulmonary artery throughout the cardiac cycle, leading to the characteristic continuous murmur.

This murmur typically sounds like a "machine-like" noise, which is distinct from the murmurs associated with other heart defects. In contrast to PDA, the murmurs produced by ventricular septal defects and atrial septal defects are usually not continuous and may vary based on the position of the patient and other factors. Congenital aortic stenosis produces a systolic ejection murmur rather than a continuous one. Therefore, the presence of a continuous machinery murmur in a child strongly points toward patent ductus arteriosus as the likely diagnosis.