D was driving drunk and struck P, a pedestrian. P sustained numerous injuries, including a compression fracture in a spinal vertebra, a concussion with intracranial bleeding, a fractured femur in her left leg, and torn cartilage in her left knee. P also sustained other injuries to her left leg, left hand, and right elbow. D admitted negligence in the operation of his vehicle but contended that P had pre-existing injuries for which he was not liable because they were not caused by his conduct. D elicited testimony that P had been treated two months prior to the accident, in November 1987, for pain in her right knee. She could not fully extend her knee at that time and was taking Motrin (a drug designed to decrease swelling of arthritic joints) on an as-needed basis. P was also prescribed Darvocet, a mild painkiller, and Chlorinol, an anti-inflammatory drug, during November 1987. Medical records from 1978 indicated that P previously complained of intermittent low back pain and discomfort with forced flexion. P had problems with her knee for as long as four years (prior to the date of the report). Another doctor indicated that P's vertebra fracture might be old. The jury also heard evidence that P's knee had some degeneration as a result of the normal aging process, that P might be predisposed to causalgia, and that P's knee surgery was delayed longer than the average person's because of her blood clotting condition. D objected to the giving of a thin-skull instruction on the grounds that the instruction told the jury that they may not refuse or reduce the amount of damages because of P's pre-existing physical ailments. P got the verdict, and D appealed.