Here, we shortly introduce the Mie-Gans theory to model the extinction of core-shell nanospheroids. We then describe the classical Lorentz approach to model the shell and discuss the appearance of the spurious peak for the simplest example of plasmonic nanoparticles: A spherical particle with Drude response. In this way, we can exemplify how the HQM that we will introduce in the following section modifies the extinction of core-shell particles.