What is the state's responsibility to preserve and preserve potentially exculpatory evidence in a criminal case?

The state's responsibility is further limited when the defendant's challenge is based on the failure to preserve potentially exculpatory evidence - that is, "evidentiary material of which no more can be said than that it could have been subjected to tests, the result of which might have exonerated the defendant." (Youngblood, supra, 488 U.S. at p. 57, italics added.) "[U]nless a criminal defendant can show bad faith on the part of the police, failure to preserve potentially useful evidence does not constitute a denial of due process of law." (Id. at p. 58; People v. DePriest (2007) 42 Cal.4th 1, 41-42.)