I am not sure that "preliminary" means the most important in this case. People tend to refer to the "primary" cause of death as the most important one. The "preliminary" cause may mean the initial, suggested cause after the initial, brief, "preliminary" inspection of the body. After a detailed inspection, the pathologist usually states his or her "final" view on the causes of death (including the "primary" cause). More context would help.

The main cause.

The key word in this sentence is preliminary which means in this case- the most important cause of.