The argument is introducing rheumatoid arthritis; improper immune response attacking joint cells that triggers hormone release, which in turn causes pain and swelling. This hormone is normally only released when there is injury or infection. A new drug will inhibit this hormone function.
This question is Most Strongly Supported (a subtype of Must Be True), while classifications are not wholly relevant, knowing what the question type asks of you is very important. In these questions, we are going fact finding for the one answer choice that is strongly supported by the information in the passage. There is no need to attempt to predict an answer, simply because there are so many possibilities for the answer choices that it is typically a fruitless exercise.
Answer choice C says that a patient treated using this medication may sustain a joint injury and be unaware of it. This is correct because the medication blocks the function of the hormone which results in pain and inflammation. If this patient no longer has a hormone function producing pain and inflammation, they can very well damage the joint and not feel any pain, nor seen any visible signs of inflammation.
I hope this helps. Please let me know if you have any other questions!