The interviewer understands that every surgery is not an elective procedure and may be a more difficult situation than others. When you are asked a question like this during an interview, it is not necessarily because the interviewer wants to hear a story of a difficult time, but more likely that he is trying to see how well you cope with difficult situations. As always, remember to protect patient confidentiality when giving an answer.
"The most difficult case I ever worked was the surgery of a 14 yr old boy who had been in an automobile accident. The driver of the car that hit him was drunk. The boy lost his leg and the other driver walked away with only a few cuts and bruises. It was a devastating event."
"I have only recently begun my career as a surgical first assistant and can't think of an unusually difficult case that I have personally assisted with. Of course, I have heard stories of difficult cases, such as people who experience the loss of a limb, or a stillbirth, but I have not yet worked any such case."