Clinician: Patients with immune system disorders are usually treated with a class of drugs that, unfortunately, increase the patient' s risk of developing osteoporosis, a bone-loss disease. So these patients take another drug that helps to preserve existing bone. Since a drug that enhances the growth of new bone cells has now become available, these patients should take this new drug in addition to the drug that helps to preserve existing bone. Which one of the following would be most useful to know in order to evaluate the clinician's argument? ['How large is the class of drugs that increase the risk of developing osteoporosis?', 'Why are immune system disorders treated with drugs that increase the risk of developing osteoporosis?', 'To what extent does the new drug retain its efficacy when used in combination with the other drugs?', 'Is the new drug more expensive than the drug that helps to preserve existing bone?']