<p>If you feel the need to explain it, I’d try to fit the explanation in the middle of the essay or in a separate addendum and say something like “I choose to withdraw from my biology class because I wanted to focus on my political science classes/extracurriculars that would contribute to my long term goals”. But then again, why bring it up? There’s nothing wrong with one W. If you did well in your other classes, they’ll assume that you just didn’t feel you had the proper background.</p>