Which term is used to describe a heart sound that can only be detected when a stethoscope is partly off the chest?

The correct choice is linked to the classifications of heart murmurs based on their intensity, which is determined by how easily they can be heard through a stethoscope. A Grade 6 murmur is characterized as a very loud murmur that can be heard even when the stethoscope is placed partly off the chest. This distinguishes it from lower-grade murmurs, which require the stethoscope to be placed directly on the chest to be detected.

Grade 4 murmurs are loud and can be felt as a thrill on the chest wall, but they are not audible off the chest. Grade 5 murmurs are also very loud and can be heard with the stethoscope at the edge of the chest, but still require it to be largely on the chest to be appreciated. Meanwhile, Grade 3 murmurs are moderate in intensity and can be heard distinctly with the stethoscope on the chest but do not possess the volume or audibility characteristics that would allow detection when partly off the chest.

This ordering of intensity and the specific requirement for a Grade 6 murmur to be detected partly off the chest highlight its unique classification and confirm its correctness as the answer.