Once each year during the first five years following the retirement of a member on a disability retirement allowance, and once in every three-year period thereafter, the board of trustees may, and upon his application shall, require any disability beneficiary to undergo a medical examination, the examination to be made at a place designated by the medical board, and to be made by the medical board or by a physician or physicians designated by such board. Should any disability beneficiary refuse to submit to the medical examination, his allowance may be discontinued until his withdrawal of the refusal, and if his refusal continues for one year all rights in and to his allowance may be revoked by the board of trustees.