  We present a detailed analysis of the single-slit optical spectrum of the
Flat-Spectrum Radio Quasar (FSRQ) B2 0003+38A, taken by the Echellette
Spectrograph and Imager (ESI) on the Keck II telescope. This classical
low-redshift FSRQ ($z=0.22911$, as measured from the stellar absorption lines)
remains underexplored in its emission lines, though its broad-band continuum
properties from radio to X-ray is well-studied. After removing the unresolved
quasar nucleus and the starlight from the host galaxy, we obtain a
spatially-resolved 2-D spectrum, which clearly shows three components,
indicating a rotating disk, an extended emission line region (EELR) and an
outflow. The bulk of the EELR, with a characteristic mass $M_{\rm EELR}\sim
10^{7}~\rm M_{\odot}$, and redshifted by $v_{\rm EELR}\approx 120$ km s$^{-1}$
with respect to the quasar systemic velocity, shows a one-sided structure
stretching to a projected distance of $r_{\rm EELR}\sim 20$ kpc from the
nucleus. The rotation curve of the rotating disk is well consistent with that
of a typical galactic disk, suggesting that the FSRQ is hosted by a disk
galaxy. This conclusion is in accordance with the facts that strong absorption
in the HI 21-cm line was previously observed, and that Na
I$\lambda\lambda5891,5897$ and Ca II$\lambda\lambda3934,3969$ doublets are
detected in the optical ESI spectrum. B2 0003+38A will become the first FSRQ
discovered to be hosted by a gas-rich disk galaxy, if this is confirmed by
follow-up deep imaging and/or IFU mapping with high spatial resolution. These
observations will also help unravel the origin of the EELR.
