  The magnetorotational instability (MRI) plays a key role for cosmic structure
formation by triggering turbulence in the rotating flows of accretion disks
that would be otherwise hydrodynamically stable. In the limit of small magnetic
Prandtl number, the helical and the azimuthal version of MRI are known to be
governed by a quite different scaling behaviour than the standard MRI with a
vertical applied magnetic field. Using the short-wavelength approximation for
an incompressible, resistive, and viscous rotating fluid we present a unified
description of helical and azimuthal MRI, and we identify the universal
character of the Liu limit Ro= -0.8284 for the critical Rossby number. From
this universal behaviour we are also lead to the prediction of higher azimuthal
wavenumber for rather small ratios of azimuthal to axial applied fields.
