What is a qualified expert in a medical malpractice case?

"A person is qualified to testify as an expert if he has special knowledge, skill, experience, training, or education sufficient to qualify him as an expert on the subject to which his testimony relates." (Evid. Code, 720, subd. (a).) " 'We are required to uphold the trial judge's ruling on the question of an expert's qualifications absent an abuse of discretion. [Citation.] Such abuse of discretion will be found only where " 'the evidence shows that a witness clearly lacks qualification as an expert.' " [Citation.]' [Citation.]" (People v. Wallace (2008) 44 Cal.4th 1032, 1062-1063.)