What category of legal expertise should I look for in an attorney to represent me for a charge levied against me for a TSA/ airport security issue?

In September, I was stopped by airport security when the body scanner showed a small utility knife blade in my cell phone protective case. I put it there in order to affix my cell phone to a cell phone mounting magnet that I have glued to my car dashboard, so I can see the GPS screen when I'm driving.This was an innocent mistake of forgetfulness, I chose the blade only because it was thin enough to fit, and still be effective. I affixed it a year before I even knew I'd be flying,and forgot about it. I have significant memory problems due to severe chemo therapy I endured in 2010, when I had stage four neck cancer. I am officially disabled, partly due to these memory problems. I had already flown twice, a few days prior and was on my way home, when this happened. I cooperated fully, provided my background as a business and community leader and a candidate for the US House of Rep. in 1992.I am concerned that if I pay the fine I'll be deemed "guilty" and have limited rights. Please help!