Shares of UnitedHealth Group Inc. UNH, -0.72% tumbled 5.3% in afternoon trade Friday, to extend its selloff to a one-year low, in the wake of Senator Bernie Sanders's unveiling of his "Medicare-for-all" plan. The health insurer's stock sank 4.3% on Thursday, to bring the combined 2-day loss to 9.4%, the biggest two-day tumble since plunged 9.4% over the 2 days ending March 3, 2009. Sanders’s plan would create a single-payer health care system to replace the current job-based and individual private health insurance system. Raymond James analyst Chris Meekins said the largest impact by far would be on health insurers. The health care sector was the only one of the S&P 500's SPX, -2.37% 11 sectors that was declining Friday. The SPDR Health Care Select Sector ETF XLV, -1.03% shed 0.9%, while the S&P 500 climbed 0.5%.