her superior officer or physical strain from serving on ACDUTRA while she was pregnant , the evidence of record does not support this contention . The medical records of the Veteran ' s son R . do show that he has a serious disability which requires constant care , but treatment records from 1986 and 1987 show that he had encephalopathy of unclear origin precipitated by tonsillitis and a fever . The Veteran herself seems unsure of the cause of her son ' s illness , as her statements made prior to filing the current claim show that she believed the illness was caused by an adverse reaction to a vaccination . The credibility of a witness can be impeached by a showing of interest , bias ,