Thanks @Yousef Darrat  . I still think the initiation is not consistent with PMT for two reasons: 

First, the beat before WQRS is beat #3, (and I assume that is the one you refer to as there is a tiny possibly pacing spike at its onset although that spike is not visible in the bottom lead) is probably "pseudofusion" rather than actual "fusion", as it looks almost identical to the underlying native beat (beat #1) and likely generated by a P wave (red arrow) seen at the end of the T wave of the previous beat (#2 ?PVC). If that is the case, then there is no source for a retrogradely conducted P wave to initiate PMT loop.

Second, if we assume beat#3 is actually a "fusion" beat, then that itself negates the possibility of retrograde conduction as the conductive system has to be fully antegradely activated to generate that "fusion" and hence cannot be activated again in retrograde direction. The only assumption will be  the presence of a concealed accessory pathway, which is probably unlikely (no information suggestive of that based on these tracings).