This week, the California Medical Assn., the largest physician group in the state with 35,000 members, accused health insurance giant Aetna Inc. of refusing to negotiate with member doctors or kicking physicians out of its insurance network as retaliation for a lawsuit filed last month. As a result, the association said, Aetna may be limiting patients' access to their regular doctors. In a sharply worded response, Aetna disagreed and said it is the one fighting on behalf of patients against some physicians' "egregious and improper billing practices" that drive up costs for consumers.