Depends on whether SD is pronounce "StructureDefinition" or "Ess-Dee". Checking style guides around the web, it seems "an SD" is the accepted usage. Finally, the rule  applies  to  acronyms  as well. If you  pronounce  a  letter  as a  letter  and it  begins  with a  vowel  sound, you  should  precede it with  an . The  consonants  with  vowel  sounds  include   f, h, l, m, n, r, s,  and  x . He flew in  an  SST.   He  fired   an  M‑1.   He  attended   an   FDA  hearing.