A patient presents with a low diastolic rumble murmur in the left lateral position with no radiation. What is the most likely condition?

A low diastolic rumble murmur, especially when appreciated best in the left lateral position, is characteristic of mitral stenosis. This condition is primarily caused by the narrowing of the opening of the mitral valve, which impedes blood flow from the left atrium into the left ventricle during diastole. The murmur typically has a rumbling quality and is best heard at the apex of the heart, especially when the patient is in the left lateral decubitus position because this enhances the sound transmission toward the stethoscope.

Mitral regurgitation generally produces a holosystolic murmur rather than a diastolic one, making it less likely in this scenario. Aortic stenosis is characterized by a systolic ejection murmur, which is also not consistent with the diastolic rumble described. Tricuspid regurgitation can produce a soft murmur, but it's typically heard during systole and may be associated with right heart failure symptoms, which does not match the description provided.

Therefore, the combination of a low diastolic murmur, best heard in the left lateral position, significantly points to mitral stenosis as the condition in question.