[Dr.Bobby]

Lynne, the rule of thumb is that you should always explain something that looks out of place or unusual, BUT in a way that doesn't make you look more out of place or unusual! Or, put another way, if you have something you feel needs explanation, you should do it in a way that doesn't raise more concern, you simply don't know your audience and what effect revealing depression will have. The goal is to be in as much control over how the reader interprets what you've written, that's the point here, to carefully use your words to leave the impression of yourself you feel will be most positive and beneficial.

So, something like "I'm not stranger to adversity, and I've faced more than I probably ever thought I would, but in so doing, I learned the value of perseverance and focus. This was especially noticeable in my senior year, which allowed me to maintain a 3.2 in spite of considerable personal hardship." This leaves a better impression than "As you can see, in my senior year I got a 3.2 because I was depressed".