What is the test for expert testimony in the context of a medical malpractice claim?

Appellants next contend that the trial court erred by rendering a decision that they failed to act within the standard of care without any expert testimony to that effect. While expert testimony is ordinarily employed to establish the standard of care, it is not in all cases essential. "The issue of attorney malpractice is in essence a question of fact similar to that involved in other professional negligence. [Citation]. Expert testimony is admissible to establish the standard of care applicable to a lawyer in the performance of the work for which he was engaged by the client, and to establish whether he has performed to the standard. Where the failure of attorney performance is so clear that a trier of fact may find professional negligence unassisted by expert testimony, then expert testimony is not required." (Wilkinson v. Rives (1981) 116 Cal.App.3d 641, 647-648.)