Several groups are challenging a massive $2.6 billion healthcare merger in Nevada. The American Medical Association, the Nevada State Medical Association and the Clark County Medical Society have filed documents opposing the merger with the Justice Department and a federal court in Washington D.C. They say the deal for UnitedHealth Group to buy Sierra Health Services would create an "unprecedented level of market concentration" in the Las Vegas Valley, thus limiting consumer choices and quality of service. Back in February, the Justice Department approved the deal, on the condition the new company divest itself of 27,000 Medicare Advantage customers in Southern Nevada to rival Humana.