  We study the criterion for the velocity and magnetic vector fields that solve
the three-dimensional magnetohydrodynamics system, given any initial data
sufficiently smooth, to experience a finite-time blowup. Following the work of
[12] and making use of the structure of the system, we obtain a criterion that
is imposed on the magnetic vector field and only one of the three components of
the velocity vector field, both in scaling-invariant spaces.
