Final answer:
Reperfusion injury is the most likely cellular process to occur in the myocardium immediately after the administration of tPA and resuscitation, as it can cause oxidative stress and cell damage upon the restoration of blood flow.
Step-by-step explanation:
The cellular process most likely to occur in the myocardium immediately following the administration of tPA after cardiac arrest and successful resuscitation is reperfusion injury. When blood flow is restored to the heart, the sudden influx can cause additional damage due to oxidative stress, inflammation, and calcium overload. Although reperfusion is necessary to salvage heart tissue, it can paradoxically cause cell death and exacerbate injury in the myocardial region that was deprived of oxygen and nutrients during the arrest.
Other processes like apoptosis (programmed cell death), fibrosis (formation of scar tissue), and hypertrophy (increase in cell size) may occur following a cardiac event, but they are typically more delayed responses compared to the immediate effect of reperfusion injury. Fibrosis and hypertrophy, for instance, are part of the heart's longer-term response to heal and compensate for the damaged tissue, while apoptosis can be triggered by the stress and damage to cells during the arrest and the subsequent restoration of blood flow.