Left ventricle.
Explanation:
Left ventricle is the chamber of the heart that has to work against the highest resistance. Because it has to pump blood into the systemic circulation. moreover it has a greater muscle mass and is more thicker than the right chambers of the heart.
Thus, the oxygen demand of left ventricle is much more than that of the other three chambers. If by any means the blood supply to the heart is not sufficient to fulfill the oxygen demand of the left ventricular cells then it would result in myocardial infarction commonly termed as heart attack
