established in accordance with 38 C . F . R . § 4 . 125 ( a ) , which provides that all psychiatric diagnoses must conform to the American Psychiatric Association ' s Diagnostic and Statistical Manual for Mental Disorders . 38 C . F . R . § 3 . 304 ( f ) . In this situation , the evidence establishes the Veteran has ( 1 ) medical evidence diagnosing the condition ; ( 2 ) the October 2002 C & P examiner determined the Veteran ' s PTSD traumatic stressor concerned his experience in Vietnam with a mortar attack ; and ( 3 ) the RO has conceded that the Veteran ' s claimed in - service stressor occurred . 38 C . F