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The ECG shows atrial fibrillation with Ashman phenomenon.
The RR interval is long between the first and second beats. This causes the effective refractory period of the right bundle to be longer than normal causing RBBB aberrancy as the third beat has come at a short RR interval thus falling within this ERP. Note that in spite of coming at a short RR interval, the fourth beat is not having aberrancy as the preceding RR is short (hence ERP is not increased after the third beat).