Single Judge Application; negative evidence; Horn v. Shinseki, 25 Vet.App. 231, 235 (2012); the examiner relied entirely on negative evidence, namely a lack of in-service or post-service treatment records, to reach his conclusion that the veteran’s back condition was not aggravated by service. R. at 374; Horn clarifies that the Secretary may not rely on an absence of evidence to rebut the aggravation prong of the presumption of soundness because doing so “effects an impermissible burden shift.” 25 Vet.App. at 239; - Veterans VA Disability Claim Community