(A) Many drugs undergoing clinical trials are intended for the treatment of conditions for which there is currently no effective treatment. This answer choice does not cast doubt on the conclusion of the argument. It does not address the argument's claim that physicians have a moral obligation to encourage suitable patients to volunteer for clinical trials.
(B) Patients do not share the physician’s professional concern for public health, but everyone has a moral obligation to alleviate suffering when able to do so. This answer choice does not address the argument's claim that physicians have a moral obligation to encourage suitable patients to volunteer for clinical trials. Furthermore, it is not clear how this claim is relevant to the argument.
(C) Usually, half the patients in a clinical trial serve as a control group and receive a nonactive drug in place of the drug being tested. This answer choice does not cast doubt on the conclusion of the argument. It does not address the argument's claim that physicians have a moral obligation to encourage suitable patients to volunteer for clinical trials.
(D) An experimental drug cannot legally be made available to patients unless those patients are subjects in clinical trials of the drug. This answer choice does not cast doubt on the conclusion of the argument. It supports the argument's claim that clinical trials are necessary before drugs can be made available to patients, but it does not address the argument's claim that physicians have a moral obligation to encourage suitable patients to volunteer for clinical trials.
(E) Physicians have an overriding moral and legal duty to care for the health and safety of their current patients. This answer choice casts doubt on the conclusion of the argument. It suggests that physicians may have a greater obligation to their current patients than to future patients who might benefit from new treatments. If this is true, then physicians may not be morally obligated to encourage their current patients to participate in clinical trials. Therefore, option E casts the most doubt on the conclusion of the argument