  We theoretically examine anisotropy of in-plane resistivity in the striped
antiferromagnetic phase of an iron arsenide superconductor by applying a memory
function approach to the ordered phase with isotropic nonmagnetic impurity. We
find that the anisotropy of the scattering rate is independent of carrier
density when the topology of the Fermi surface is changed after the
introduction of holes. On the other hand, the anisotropy of the Drude weight
monotonically decreases reflecting the distortion of the Dirac Fermi surface
and eventually leads to the reverse of anisotropy of resistivity, being
consistent with experiment. The origin of the anisotropy is thus attributed to
the interplay of impurity scattering and anisotropic electronic states.
