Here is the crucial exerpt:

1396p (d) Treatment of trust amounts:  .......................................................................

• (4) This subsection shall not apply to any of the following trusts:
  • (A) A trust containing the assets of an individual under age 65
    who is disabled (as defined in section 1382c(a)(3) of this title)
    and which is established for the benefit of such individual by a
    parent, grandparent, legal guardian of the individual, or a court
    if the State will receive all amounts remaining in the trust upon
    the death of such individual up to an amount equal to the total
    medical assistance paid on behalf of the individual under a State
    plan under this subchapter.