What constitutes a denial of the right to present a defense?

( Washington v. Texas (1967) 388 U.S. 14, 17-18, 87 S.Ct. 1920, 18 L.Ed.2d 1019 ( Washington ).) Under federal law, a denial of the right to present a defense occurs when the exclusion of the evidence infringes "upon a weighty interest of the accused." ( United States v. Scheffer (1988) 523 U.S. 303, 308, 118 S.Ct. 1261, 140 L.Ed.2d 413.) A weighty interest of the defendant is infringed when "[t]he exclusions of evidence ... significantly undermined fundamental elements of the accuseds defense." ( Id. at p. 315, 118 S.Ct. 1261.)