38 USC 5109 - Independent medical opinions

(a) When, in the judgment of the Secretary, expert medical opinion, in addition to that available within the Department, is warranted by the medical complexity or controversy involved in a case being considered by the Department, the Secretary may secure an advisory medical opinion from one or more independent medical experts who are not employees of the Department.
(b) The Secretary shall make necessary arrangements with recognized medical schools, universities, or clinics to furnish such advisory medical opinions. Any such arrangement shall provide that the actual selection of the expert or experts to give the advisory opinion in an individual case shall be made by an appropriate official of such institution.
(c) The Secretary shall furnish a claimant with notice that an advisory medical opinion has been requested under this section with respect to the claimants case and shall furnish the claimant with a copy of such opinion when it is received by the Secretary.