The stimulus begins by telling us that scientists believe they have found an explanation for a problem. What is the problem? Some patients who have not been taking penicillin have developed penicillin-resistant bacteria.
What is the explanation? The genes which provide a resistance to penicillin are close to the gene which provides immunity to mercury poisoning. Therefore, the patients' exposure to mercury through their dental fillings kills off bacteria which lack the immunity gene and allow bacteria with both the mercury-immunity and penicillin-resistance genes to flourish.
The question stem asks us about an unstated assumption i.e. that some people who take penicillin develop penicillin-resistant bacteria. This assumption works with the first sentence to outline our problem. It makes sense that some people who have taken penicillin may have developed bacteria which are resistant to it. The problem arises when we learn that some people who have not been taking penicillin also develop these resistant bacteria. This is directly restated by answer choice (C), making it our correct answer.
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