What is the test for determining whether a health official had requisite knowledge of a substantial risk of harm?

Whether a defendant had requisite knowledge of a substantial risk of harm is a question of fact. Farmer, 511 U.S. at 842. Thus, liability may be avoided by presenting evidence that the defendant lacked knowledge of the risk and/or that his response was reasonable in light of all the circumstances. See Farmer, 511 U.S. at 844-45; see also Simmons v. Navajo County Ariz., 609 F.3d 1011, 1017-18 (9th Cir. 2010) (requiring official be subjectively aware of serious medical need and fail to adequately respond to need to establish deliberate indifference).