42 CFR §422.590(g)(2)

(g) Who must reconsider an adverse organization determination.

(2) When the issue is the MA organization’s denial of coverage based on a lack of medical necessity (or any substantively equivalent term used to describe the concept of medical necessity), the reconsidered determination must be made by a physician with expertise in the field of medicine that is appropriate for the services at issue. The physician making the reconsidered determination need not, in all cases, be of the same specialty or subspecialty as the treating physician.