Difficult. Lawyers operate under the presumption that we would all live forever were it not for the wrongful interventions of medical practitioners. They only have to demonstrate damage and appeal to a jury of the loosest connection between the intervention and the 'bad' outcome. Negligence is hinted at, but rarely is definitive. In medmal there is lots of money and little justice.
Reasonable care. . Best is to let an attorney have expert witnesses testify as to the reasonableness of care and whether the care was the standard of care for the medical specialty involved. This will include any duty owed to the patient for care, breach of that duty, whether the breach of the duty is a direct causation of the alleged malpractice, and the damages claimed. If both negligence, and damages related to the negligence cannot be proved, there is no medical malpractice case.