As a general rule, with any CR question (strengthen, weaken, evaluate, etc.) our job is to closely examine the assumptions underlying the connection between the evidence and the conclusion (aka the inference).
In many cases, attacking the evidence or the inference is enough to do the job. For example, if I state, "he committed the crime because his fingerprints were at the scene," then I can negate that argument one of three main ways:
1) ATTACK THE EVIDENCE. If you can prove that his fingerprints weren't at the scene, then this should definitely help--although it is still possible to commit a crime without leaving fingerprints, attacking the evidence can be an effective weakener in most cases.
However, the GMAT is too difficult of a test to give us an answer choice this easy in most cases. Also, in the stem we are told that the evidence (patients' motivations) is irrelevant since doctors have the final say. In other words, the stem argument tells us that the evidence is not closely tied to the conclusion, so attacking the evidence doesn't work here.
2) ATTACK THE ASSUMPTION / INFERENCE BY POINTING OUT ALTERNATE EXPLANATIONS. This is a big one. I might want to consider whether someone who DID leave fingerprints might not have committed the crime, or who else might have left fingerprints there.
STRENGTHEN example: his were the only fingerprints in the house.
WEAKEN example: there were many fingerprints at the house.
3) ATTACK THE CONCLUSION. If you can prove that the crime was never committed, then that would of course do the job, too...but the root of GMAT CR lies in mastering #2.
The problem with Choice B is that a patient's motivations and requests, while a relevant factor, do not directly pertain to the conclusion (the physician's behavior in prescribing inappropriate medications) nearly as much as does the physician him or herself, since we are told in the stem that the doctor is in ultimate charge of that decision.
-Brian