We employ the density matrix renormalization group to construct the exact
time-dependent exchange correlation potential for an impurity model with an
applied transport voltage. Even for short-ranged interaction we find an
infinitely long-ranged exchange correlation potential which is built up
{instantly} after switching on the voltage. Our result demonstrates the
fundamental difficulties of transport calculations based on time-dependent
density functional theory. While formally the approach works, important
information can be missing in the ground-state functionals and may be hidden in
the usually unknown non-equilibrium functionals
