MDX Help - Looking for 'current measure'

If I were to use the following MDX:

   SUM( DESCENDANTS( [Accounts].[Accounts].&[12345], 1), [Measures].[Begin Balance])

I would be aggregating the begin balances of the children of the specified account.  What I want to know is how to do this without specifying the measure.  That is, if the user is looking at begin balances, then it aggregates begin balances.  If the user is looking at End Balances, then it aggregates End Balances.

  I don't know if there is a something like the CurrentMember function for Measures, or if there is a better way to write the MDX and leave off the Measure.  Oh, and if this makes a difference, the Aggregation Type for the dimension that I want to do this with is SUM, so I might not even need to use the SUM function.

Thanks,

Scott