Which of the following rhythms is considered a lethal arrhythmia?

Ventricular fibrillation is classified as a lethal arrhythmia because it results in chaotic electrical activity in the heart, preventing effective contraction and therefore, effective blood circulation. In this state, the heart's ventricles quiver instead of contracting in a coordinated manner, which can lead to sudden cardiac arrest if not resolved promptly.

In the case of ventricular fibrillation, immediate intervention, such as cardiopulmonary resuscitation (CPR) and defibrillation, is critical to restore a normal heart rhythm and prevent death. This arrhythmia can occur in the context of a heart attack, electric shock, or underlying heart disease, making it an emergency situation.

Other arrhythmias listed—like atrial fibrillation, bradycardia, and premature ventricular contractions—while they may cause complications or require management, do not typically result in immediate life-threatening situations like ventricular fibrillation does.