I was doing the Medical Mystery quest from Dr. Zed to investigate a new type of weapon being used, and it involved finding, killing and then looting Dr. Mercy. However, when I get to the part where I'm supposed to loot Dr. Mercy's body, it won't let me interact with his body to loot it. The quest marker shows a spot to go near the entrance of the cave (i.e., not where his body is), but there's nothing there, either.
How can I loot him to complete the quest?