  We study the anomalous Hall effect arising from the altermagnetic order and
spin-orbit interaction in doped FeSb$_2$. To investigate the anomalous
transport, we have constructed a tight-binding model of FeSb$_2$. We separately
considered the constraints imposed on the model parameters by the spin symmetry
group and magnetic symmetry group at zero and finite spin-orbit interaction,
respectively. The resulting model includes the effect of exchange splitting and
is applicable at both zero and finite spin-orbit interaction. In the case of
spin symmetry, the analysis covers the spin-only subgroup arising from
collinear magnetism, as well as non-trivial symmetry elements. This allows us
to explore changes in the hopping amplitudes as symmetry is reduced by
spin-orbit interaction from the spin group to the magnetic group. While the
anomalous Hall effect is forbidden by spin symmetry, it is allowed by the
symmetries of the magnetic group. The intrinsic Hall conductivity is shown to
vanish linearly with spin-orbit interaction. This non-analytic behavior is
universal to altermagnets. It originates from the singularity of the Berry
curvature localized along lines on a Fermi surface confined to symmetry planes.
These planes host spin degeneracy protected by spin symmetry, which is lifted
by spin-orbit interaction.
