  We compute the cross section for the charged-current reaction ${}^{12}{\rm
C}(\nu_{\mu},\mu^{-})$ near threshold using a relativistic mean-field
formalism. A reduced value of the nucleon mass in the medium --- coupled to the
finite muon mass --- leads to a 30\% reduction in the cross section relative to
a free Fermi-gas estimate. Isovector RPA correlations, which are strongly
repulsive at these momentum transfers, reduce the cross section by an
additional 15-30\%. Hence, relativistic nuclear-structure effects can account
for the more than a factor-of-two reduction in the cross section recently
reported by the LSND collaboration.
