The diagram below, together with its reversed fermion flow diagram counterpart, collectively sum to zero due to Furry's theorem. I just wanted to understand why this is the case. 1)There are two gluons and one photon attached to the internal line so this is 3 coupled vector current insertions (1 QED and 2 QCD). 2) The blob represents a hadronic state. Why does the diagram contain a fermion loop coupled to the 3 vectors? I thought the blob always stood for arbitrary complicated soft dynamics that we are naive about and just 'put under the rug' so I don't see why it necessarily must be a simple closure of the fermion line shown.