Which of the following is a characteristic of Class I antidysrhythmics?

Class I antidysrhythmics are primarily known for their action of blocking sodium channels. This characteristic is fundamental to their mechanism of action, as it inhibits the influx of sodium ions during the depolarization phase of the cardiac action potential. By blocking these channels, Class I agents can effectively stabilize myocardial cell membranes, reduce excitability, and slow conduction velocity, which is essential in the management of various dysrhythmias.

The other characteristics mentioned play a role in the function of different classes of antidysrhythmics or are not the primary effects associated with Class I agents. Prolonging the refractory period is more characteristic of certain Class III agents, while depression of automaticity is a broader effect seen in multiple classes, including some Class II agents (beta-blockers). Increasing calcium influx is specific to certain Class IV agents (calcium channel blockers) and does not apply to the mechanism of Class I antidysrhythmics. Thus, blocking sodium channels is the defining feature that distinguishes Class I antidysrhythmics in their therapeutic role.