I definitely have the same problem and a lot of others have the same complaint- there're often two answers that seem extremely similar, and its usually a nitpicky detail you may not have caught in the passage that determines it. One thing I've started using is asking myself- are either of these two stretching something the author said a little too far, b/c that sometimes happens. They both sound feasible but one might be slightly extreme. Also, make sure the answer comes from the same paragraph as the stuff the question is asking about (unless of course its a general question about the whole piece). Its the fact that there are usually 2 answers so close that makes the MCAT so hard- we'd all be doing really well if every question had a clear cut answer and none to complicate matters.