My Grammar Theory

This is my first blog, as I'm usually late stepping into internet phenomenon. I think phenomenon must be a singular and plural word like deer, fish, and sheep- because when I tried to add an "S", the spell check didn't let me.  Speaking of English and grammar, I have a certain theory - I have a lot of my own theories, but this particular one bugs me when english teachers disagree because I want it to be right.  Here it is: Do you know the rule where you put "a" in front of words that start with a consonant like "a cat", and you put "an" in front of a word that starts with a vowel like "an apple".  Well I think that rule was created because of how it sounds- "A apple" and "An cat" sounds just horribly wrong.... But here is where I believe there is an exception to that rule- Sometimes in sentences, you may write an abbreviation like M.D., for medical doctor.  Well if you say "M", it sounds like "EM". Therefore, I would say that it is only right to say "an M.D." Doesn't that sound more correct than "a M.D."? I found an exception on the other side of the coin as well, using "a" before a vowel.  "A unique person". "Unique" sounds like "Yunique". According to the actual rule, you would have to write "an unique person". wrong wrong wrong.  If anyone has a good argument against this, let me know! Lemontime signing off