We show that modified Newtonian dynamics (MOND) is equivalent to assuming an isothermal dark matter density profile, with its density related to the enclosed total baryonic mass. This density profile can be deduced by physical laws if a dark matter core exists and if the baryonic component is spherically symmetric, isotropic, and isothermal. All the usual predictions of MOND, as well as the universal constant a0, can be derived in this model. Since the effects of baryonic matter are larger in galaxies than in galaxy clusters, this result may explain why MOND appears to work well for galaxies but poorly for clusters. As a consequence of the results presented here, MOND can be regarded as a misinterpretation of a particular dark matter density profile. Copyright © 2013 American Physical Society.