While I cannot find any data to back me up, I believe you would say, “Bob holds an M.A. in Management” if you are using the initials. However, you would say, “Bob holds a Master of Arts in Management” if you were using the words. In thinking about this, I suspect we use an in front of the consonant letters F, L, M, N, R, S, and X (as well as in front of all vowels) when we are referring to them as individual letters for ease of speaking. Think of the television show, “Wheel of Fortune,” where contestants say, “I’d like to buy a B” or “I’d like to buy an N.”