What is the standard of expert testimony used in a medical malpractice case?

This court has "set forth four criteria to determine the helpfulness of expert testimony: (1) qualified expert; (2) proper subject; (3) conformity to a generally accepted explanatory theory; and (4) probative value compared to prejudicial effect." United States v. Christophe, 833 F.2d 1296, 1299 (9th Cir.1987). In Christophe, this court also held that the type of expert testimony Mixon seeks to use does not conform to a generally accepted explanatory theory. Id.