  We estimate the highest energy of proton diffusively accelerated by shock in
knot A1 of the jet in luminous nearby quasar 3C 273. Referring to the recent
polarization measurements using very long baseline interferometry (VLBI), we
consider the shock propagation across magnetic field lines, namely,
configuration of the oblique shock. For larger inclination of the field lines,
the effects of particle reflection at the shock front are more pronounced, to
significantly increase acceleration efficiency. The quasiperpendicular shock
turns out to be needed for safely achieving the proton acceleration to the
energy above 100 EeV (10^20 eV) in a parameter domain reflecting conceivable
energy restrictions.
