In Williams v. Patterson, 681 A.2d 1147 (D.C. 1996), which was actually a legal malpractice case, no medical expert had been called to testify "that the injuries the plaintiff suffered (and for which her attorney assertedly neglected to take action) stemmed in part from the automobile accident." 681 A.2d at 1150. In explaining why expert testimony was necessary to establish that causation, the Court did not imply that an expert would have to quantify -- in percentage terms -- the degree to which the accident worsened the prior condition, so long as the expert could "differentiate between a present medical condition and a preexisting one in evaluating the causal role of the intervening accident." Id.