How does being a doctor help John with his unofficial investigation of Karen's death?

As a doctor, John has unusual access to information and evidence other people would not be able to get near. This helps him put together the pieces of the puzzle regarding Karen's death. He is able to watch her autopsy being performed, he can take a sample of her blood and have it tested by another medical expert, and he can review her chart. Being a doctor helps him immensely with his unofficial investigation.