  We show that the interference between scattering by impurities and by
critical spin fluctuations gives rise to anisotropic transport in the
Ising-nematic state of the iron pnictides. The effect is closely related to the
non-Fermi liquid behavior of the resistivity near an antiferromagnetic quantum
critical point. Our theory not only explains the observed sign of the
resistivity anisotropy $\Delta\rho$ in electron doped systems, but also
predicts a sign change of $\Delta\rho$ upon sufficient hole doping.
Furthermore, our model naturally addresses the changes in $\Delta\rho$ upon
sample annealing and alkaline-earth substitution.
