Final answer:
The most significant symptom of LV free wall rupture after a myocardial infarction is cardiac arrest due to cardiac tamponade, which requires immediate medical intervention.
Step-by-step explanation:
The significant symptom resulting from post-myocardial infarction (MI) complication of left ventricular (LV) free wall rupture is cardiac arrest, which is a sudden loss of heart function leading to a loss of consciousness and absence of breathing. When LV free wall rupture occurs, it leads to rapid hemodynamic collapse due to the loss of blood into the pericardial space, causing cardiac tamponade. As this is a catastrophic event, rapid intervention is critical for survival, however, mortality rates are exceedingly high even with prompt medical attention.