Hrm... maybe I wasn't clear in my original post. What I meant is that the one bad grade didn't really affect my overall GPA, but when broken down into GPA by year, the final year's GPA definitely is a bit lower than the previous years'.
I'm not quite sure what to say about it other than the professor's final was completely irrelevant to the class and I was just stumped even though I did well in the rest of the class. Her grading scheme was also very harsh and she's well known to be the prof that gave many straight-A students their only non-A grade.
None of the above reasons are good to put in an optional sentence, IMHO, because they sound like whining. But I don't really have any good reasons on why I got the bad grade other than what I stated already.
To make it more complicated, I'm already using the optional essay area to discuss my community involvement activities, so it'll be weird to have 2 sentences for the grade, and then the rest on something completely different...
Thus my dilemma.