The federal government is illegally denying thousands of chronically ill Americans needed therapies and medical services, five national organizations charged Tuesday in a lawsuit filed in U.S. District Court in Vermont. The class action lawsuit is the first to mount a broad challenge to a controversial Medicare policy requiring that patients achieve demonstrable improvements in functioning in order to qualify for physical, speech and occupational therapy and skilled nursing care. If such improvements are absent, Medicare often will refuse to pay for services and medical providers will cut them off. Critics say that, as a result, stroke survivors, accident victims and people with Parkinson's disease or multiple sclerosis are wrongfully deprived of care that could help them remain independent, maintain current abilities and prevent deterioration.