What is the test for expert testimony in a medical negligence case?

As D.A.Wilson J. observed in Tahir v. Mitoff, 2019 ONSC 7298, at para. 40: Medical negligence cases, along with other professional negligence cases, in the vast majority of cases, require expert evidence to assist the trier of fact to determine the standard of care that is applicable, whether there has been a breach of that standard and whether the damages that result are caused by the negligence. In making these determinations, the trier of fact must necessarily weigh conflicting testimony from similarly qualified experts and assess the proper weight to be given to the expert opinions.