Medicare Managed Care Act of 1990 - Amends title XVIII (Medicare) of the Social Security Act to provide for the certification of a Medicare supplemental policy which would meet the National Association of Insurance Commissioners' Model Standards except that its benefits are restricted to items and services furnished by certain entities if: (1) full benefits are provided for items and services through a network of entities which have contracted with the policy issuer; (2) full benefits are provided for items and services furnished by other entities when such items and services are necessary and cannot reasonably be obtained through the network; (3) the network offers sufficient access; and (4) the policy issuer has arranged ongoing quality assurance of the items and services furnished through the network.