Usually in the interview they will bring it up. I had a C in one of my ochem courses because I was devoting a lot of time to my research job and neglecting my studies for that class. They noticed that I was performing well in my academic career except for that time and wanted to know why. I told them that I was determined to show that I learned the material well and proved it with my DAT score in that section. They were pleased.
I didn't explain it in my essay, I was just prepared to address it if it did come up.