What condition is indicated by a mid-systolic murmur that increases in intensity when standing?

A mid-systolic murmur that increases in intensity when standing is characteristic of hypertrophic cardiomyopathy (HCM). This condition involves a thickening of the heart muscle, which can obstruct blood flow out of the left ventricle, especially during periods of increased demand, such as standing or exercise. When a person stands, the decreased venous return leads to less filling of the left ventricle and greater obstruction, which intensifies the murmur.

HCM is particularly known for this characteristic behavior of its murmur, distinguishing it from other types of murmurs that do not exhibit such changes with posture. For example, aortic stenosis typically presents with a fixed intensity murmur that does not change significantly with position. Mitral regurgitation can cause a holosystolic murmur that might not vary with posture in the same manner. A venous hum is typically related to blood flow in the veins and does not generally present as a mid-systolic murmur related to dynamic changes in the heart’s output. Therefore, the characteristics of the murmur in hypertrophic cardiomyopathy align with the clinical presentation noted in the question.