In which situation is immediate defibrillation indicated?

Immediate defibrillation is indicated in the situation of ventricular fibrillation. This is because ventricular fibrillation is a life-threatening arrhythmia characterized by chaotic electrical activity in the heart, which prevents effective pumping of blood. The only effective treatment to restore a normal rhythm in a patient experiencing ventricular fibrillation is rapid defibrillation, which delivers an electrical shock to the heart, allowing it to reset and potentially regain a normal rhythm.

Heart failure, asystole, and stable atrial fibrillation do not warrant immediate defibrillation. Heart failure involves the heart's failure to pump effectively, requiring different management strategies such as medications or advanced heart failure care. Asystole, a state where there is no electrical activity in the heart, typically requires different interventions like medication and advanced cardiac life support rather than defibrillation. Stable atrial fibrillation may not require immediate intervention at all, as it can often be managed with rate control or rhythm control strategies depending on the patient's overall condition. Thus, the correct answer emphasizes the urgency of defibrillation specifically for ventricular fibrillation.