In Combs v. Norfolk and Western Ry. Co., 256 Va. 490, 496, 507 S.E.2d 355, 358 (1998), a witness was qualified as an "expert witness on the subject of biomechanical engineering." Combs, 256 Va. at 494, 507 S.E.2d at 357. The witness had a "bachelor's degree in mechanical engineering, a master's degree in medical science, and a Ph.D. in fluid, thermal and aerospace sciences, with a specialization in biomedical engineering." Id. "He is a professor of engineering, science, and mechanics and is the director of the biomedical engineering program at Virginia Polytechnic Institute and State University." Id. Although the witness "had completed all the academic work required for a medical degree, he had not completed a medical internship or residency and was not licensed to practice medicine." Id. Combs held that an expert in "the field of biomechanical engineering" could not testify as to medical causation even though he had completed the "academic work required for a medical degree" because "the fact that a witness is an expert in one field does not make him an expert in another field, even though the two fields are closely related." Combs, 256 Va. at 496, 507 S.E.2d at 358. Thus, the Court held that an expert in "the field of biomechanical engineering" was not an expert in "medical causation." Id.