Hi

I am 13, 5'4", and 107 lbs.  I recenty got over an almost-but-not-quite eating disorder, and when my doctor realizd this, I had blood tests done.  I was not allowed to exercise (to allow creatinine levels in my blood to go back to normal), and I now need to gain about 3.5 lbs.  However, I don't want to gain fat (I don't need to).  I need to gain muscle.  How can I do this??

Hmm... Well, if you are allowed to exercise now, then I suggest (don't laugh!) joining gymnastics. It may seem a bit . . . ummm. . . weird, but it helped me gain a bit of muscle. When you first start, you may notice that you are losing weight, but when you get into it you will probably put on some muscle weight. Don't take my opinion on it yet; ask your doctor if it would be a good idea for you first! 

I'm very sorry if my answer isn't helpful. I am completely new here!

Thanks for the tip!  I'll ask my doctor about it next time I go!