The doctor says that the patient has either disease X or Y, but there's no way to tell which one it is. She then says that since there's a treatment for Y but not X, they must act on the assumption that the patient has disease Y.
The question asks, "The physician's reasoning could be based on which one of the following principles?"
We're looking for some type of principle that matches the doctor's reasoning. We know that disease X has no treatment, but disease Y does. The doctor then says we've got to assume that the patient has disease Y, which has a treatment. In other words, the doctor is basically saying that if there is a medical issue with two possible causes, and only one of these two causes has a treatment, it's best to assume that the problem is caused by the disease that has a treatment.
In looking at the answer choices, we're searching for the one that best matches our anticipation.
(D) says, "When success is possible only if a circumstance beyond one's control is favorable, then one's strategy must be based on the assumption that this circumstance is in fact favorable."
We know from the stimulus that the doctor has no control over whether or not the patient has disease X or disease Y. Of these two possibilities of disease for the patient, disease Y is the only favorable one because it is the only one with a treatment. As a result, the doctor makes the assumption that her patient has disease Y. The principle outlined in (D) matches the principle of the doctor's argument well, so this is the correct answer choice.
Does this make sense? Let us know if you have any questions!
