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 who has not yet attained age sixty to undergo a medical examination, such examination to be made at a place designated by the medical board, the examination to be made by the medical board or by a physician or physicians designated by such board. Should any disability beneficiary who has not attained the age of sixty refuse to submit to such medical examination, his allowance may be discontinued until his withdrawal of such refusal and should his refusal continue for one year all rights in and to his pension may be revoked by the board of trustees.