a question arises as to whether it pre - existed service . Gilbert v . Shinseki , 26 Vet . App . 48 ( 2012 ) . Under such circumstances , VA is required to prove by clear and unmistakable evidence that a disease or injury manifesting in service both preexisted service and was not aggravated by service . Id . In determining whether service connection is warranted for a disability , VA is responsible for determining whether the evidence supports the claim or is in relative equipoise , with the Veteran prevailing in either event , or whether a preponderance of the evidence is against the claim , in which case the claim is denied . 38 U . S . C . A . § 5107