What is the test for drawing an adverse inference in a medical malpractice case?

The judge in the case at bar began her analysis by stating, correctly, that the approach to drawing an adverse inference engages the court’s discretion and requires the trier of fact to consider the following factors: a) Whether there is a legitimate explanation for failing to call the witness; b) Whether the witness is within the exclusive control of the party or is equally available to both parties; and c) Whether the witness has key evidence to provide or is the best person to provide the evidence in question. (At para. 55, citing McIlvenna v. Viebig 2012 BCSC 218 at para. 71, aff'd on other grounds at 2013 BCCA 411.)