UnitedHealth Group's stock ($UNH) plunged over 10% following the Trump administration's proposal to maintain flat Medicare reimbursement rates for insurers. This significant market reaction comes shortly after Rep. Kevin Hern, a member of the House Subcommittee on Health, sold up to $500,000 worth of UnitedHealth stock last month. The proposed reimbursement rates have raised concerns among investors about potential impacts on insurer revenues.