The rule for using the articles a and an is quite simple. Use a before words that begin with a consonant sound and an before words that begin with a vowel sound. Notice that I am not referring to the letter which a word begins with, but the sound. Try this out. Say M.B.B.S. Now say the word monkey. Do you hear a difference between the m in monkey and MBBS? The letter m when said in isolation, as it is done in the case of MBBS and MP, begins with a vowel sound. That's the reason why you use the article an.
But the same letter in words like monkey and man begins with a consonant sound. This also explains why the word university although it begins with the letter u does not take the article an before it. The word does not begin with a vowel sound; the u in university sounds like the y in yes.
COURTESY : The Hindu (The National News-Paper) - India