Final answer:
The most likely cause of a new holosystolic murmur heard at the apex with radiation to the axilla post-MI is mitral valve regurgitation, typically due to papillary muscle dysfunction or rupture following the infarction.
Step-by-step explanation:
The most likely cause of the post-myocardial infarction (MI) new holosystolic murmur heard best at the apex with radiation to the axilla is mitral valve regurgitation. Holosystolic murmurs, or pansystolic murmurs, are indicative of blood flow from a high pressure chamber to a lower pressure chamber throughout systole, as seen in mitral valve regurgitation. This particular type of murmur is best heard at the apex of the heart and can radiate towards the axilla. In the context of a recent MI, papillary muscle rupture or dysfunction can lead to mitral regurgitation as these muscles are critical to the proper function of the mitral valve. Other valve regurgitations, such as aortic valve regurgitation, tricuspid valve regurgitation, and pulmonary valve regurgitation, tend to have different auscultatory features and are less likely to present as a holosystolic murmur radiating to the axilla following an MI.