a or an

Can anyone clarify for me if you use a or an before an acronym.  I am totally confused.  For example,

a MRA or an MRA

Thanks!

• a or an - angelmt
  • Per AAMT BOS ... "an" (see message) - MT50
    • wondering then if its an apnea hypopnea index... just wondering... n/m - qtxan
      • Yes it is - MT50

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