I’m wondering if orthodontic residencies will see that this one course caused a decrease in my GPA and rank, and if this will make them more understanding of why my rank decreased?
You know the answer to this. Of course they are going to see where your GPA and more importantly class rank are at. Are they going to give you a break "because it was only one class"? Most likely not. 6.5 credits is a lot and should be a priority to do well in. But no point in crying over spilled milk, try your best and finish strong. Make up for that lapse.