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During the commissioning of {\em JWST}, the Medium-Resolution Spectrometer (MRS) on the Mid-Infrared Instrument (MIRI) observed the planetary nebula SMP LMC 058 in the Large Magellanic Cloud. The MRS was designed to provide medium resolution (R = $\lambda$/$\Delta\lambda$) 3D spectroscopy in the whole MIRI range. SMP LMC 058 is the only source observed in {\em JWST} commissioning that is both spatially and spectrally unresolved by the MRS and is a good test of {\em JWST's} capabilities. The new MRS spectra reveal a wealth of emission lines not previously detected in this planetary nebula. From these lines, the spectral resolving power ($\lambda$/$\Delta\lambda$) of the MRS is confirmed to be in the range R $=$ 4000 to 1500, depending on the MRS spectral sub-band. In addition, the spectra confirm that the carbon-rich dust emission is from SiC grains and that there is little to no time evolution of the SiC dust and emission line strengths over a 17-year epoch. These commissioning data reveal the great potential of the MIRI MRS for the study of circumstellar and interstellar material., Comment: 12 pages, 7 figures, 3 tables, accepted MNRAS