Physician: The patient is suffering either from disease X or else from disease Y, but there is no available test for distinguishing X from Y. Ttherefore, since there is an effective treatment for Y but no treatment for X, we must act on the assumption that the patient has a case of Y.
The physician's reasoning could be based on the principle that when only one strategy carries the possibility of success, circumstances must as much as possible be changed to fit this strategy.