The $^{54}$Fe nucleus was populated from a $^{56}$Fe beam with an energy of $E/A$=500 MeV. The internal decay of the 10$^+$ metastable state via $\gamma$-ray emission was observed. The structure of this isomeric state has to involve at least four unpaired nucleons, therefore it cannot be populated in a simple two-neutron removal reaction from the ground state of $^{56}$Fe, and we suggest that it was populated via the decay of the $\Delta ^0$ resonance into a proton. This process allows the population of four-nucleon states, such as the observed isomer. Therefore, the population of the four-particle 10$^+$ isomer in $^{54}$Fe is a consequence of the quark structure of the nucleons. The possible use of this reaction mechanism for producing exotic nuclei is discussed.