Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia and is frequently initiated by ectopic activity originating from the pulmonary veins (PVs). Shortened effective refractory periods (ERPs) in these structures have been reported and identified as key risk factors for AF initiation in clinical studies. In this work, a three-dimensional atrial model was developed to investigate the impact of PV ERP relative to the surrounding atrial tissue ERP (baseline ERP: 220 ms) on AF inducibility. Single premature stimuli were delivered within the PV region, and the resulting arrhythmic responses were evaluated across a range of coupling intervals and ERP configurations. The results demonstrate that when PV ERP is reduced to 150 ms or below, single ectopic beats are sufficient to induce sustained reentrant activity leading to AF. In contrast, longer PV ERP values do not result in arrhythmia initiation. These findings indicate that the ERP gradient between the PVs and atrial tissue plays a key role in enabling the transition from a single ectopic beat to sustained reentrant activity.