Physician: The patient is suffering either from disease X or else from

There is an effective treatment for Y but there is not one for X
→
We should assume that the patient has Y

So what is going on here? Well, we are assuming that, when there is no possibility of success in one instance, we should assume the other instance - the one that does have the possibility of success.

(A) This is what I originally picked but then when I came back I erased it and went with the correct answer. This is incorrect basically because it is unsupported. This is a big thing with principle questions. If we are deriving a principle then it should be almost (or in some cases, definitely) 100% supported. This is not supported because of the phrase more important. We are given no indication that "treating the diseases" is more important than "determining which of the two diseases that patient has." Maybe - and I'd actually presume that this is so - the former is much more important than the latter but the latter is impossible to figure out!

(B) The "circumstances beyond a decision maker's control" is the disease itself. While the physician is the decision maker. The doctor cannot control what disease the patient has but he/she must absolutely decide what to do. However, this is wrong because it is a reversal of the conclusion. The argument says that "we must act on the assumption of Y" - the more favorable one. However, I would argue that (B) would be right if the second clause was substituted with something about the "favorable" being assumed.

(C) We know nothing about testing assumptions and testing assumptions is not even mentioned in the argument. This is hard to eliminate from reason alone because it is so out of scope. It is just so far off.

(E) Here is the simple way to eliminate (E): the argument talks about changing strategies to fit the circumstances. The circumstances are unchangeable! Whether or not the patient has X or Y...it is unchangeable! Thus the argument is not changing the circumstances, it is just applying the strategy.

(D) This is correct. Let's dissect what it is saying.

(Success is possible → circumstance is favorable) → strategy must be based on the assumption that the circumstance is favorable

Confusing, right? Let's do a little adjustment, contrapositive style.

(circumstance is not favorable → success is not possible) → strategy must be based on the assumption that the circumstance is favorable.

So do you see what is happening here? "The circumstance is not favorable"? Well yes! The circumstance is that the physician doesn't know if the patient has X or Y. That isn't favorable because only Y has an effective cure. So the premise matches up!

Now onto the conclusion: "strategy must be based on the assumption that the circumstance is favorable." This is exactly what is happening with the conclusion of the argument! The strategy is being based on the "the assumption that the patient has a case of Y"