PrepTest 134, Section 1, Question 22
Microbiologist: Because heavy metals are normally concentrated in(54 more words not shown)
Which one of the following,(7 more words not shown)
- AMost bacteria that(12 more words not shown)
- BBacteria that live(27 more words not shown)
- CAntibiotic resistance of(19 more words not shown)
- DSewage sludge that(13 more words not shown)
- EMany kinds of(16 more words not shown)
Reading the argument
What if there are antibiotics in the sludge too?
The bacteria that survive among heavy metals have developed resistance to heavy-metal poisoning. I can follow that part of the account. Then we learn that the same bacteria resist antibiotics, and the microbiologist attributes that resistance to exposure to the metals.
But finding both kinds of resistance together doesn't tell us that metal exposure produced the antibiotic resistance. Another feature of the sludge might matter. If antibiotics are present alongside the metals, exposure to those antibiotics could supply an alternative explanation.
What we’re looking for
The question asks us to strengthen the microbiologist's explanation, so we need evidence pointing back to the metals. I'd look for something that helps separate their contribution from the other features of the sludge. We don't have to establish every biological step; we need a reason to take the proposed cause more seriously.
Working the answers
- A
This links the absence of one resistance with the absence of the other, so it gives another association. But it doesn't isolate exposure to heavy metals as the cause of antibiotic resistance. Something else could account for the two resistances appearing together or being absent together. The causal question remains.
- BCorrect
Now we have a useful comparison: bacteria in sludge without heavy metals generally have neither resistance, even though the sludge is otherwise similar to normal sewage. If another ordinary feature of sewage were responsible for the antibiotic resistance, we'd expect that feature still to be present in this comparison. Instead, lacking the metals goes along with lacking the resistance.
That makes the metals a more plausible contributor. B doesn't describe removing metals from the very same bacteria and watching them lose resistance; it compares bacteria living in otherwise similar environments. Nor does it prove the mechanism. It strengthens by reducing the appeal of explanations based on other features of the sludge.
- C
This points in the other direction: antibiotic resistance contributes to heavy-metal resistance. The microbiologist proposes that exposure to heavy metals promotes antibiotic resistance. Showing the first relationship doesn't establish the second. C doesn't rule out both operating, but it supplies a different explanation for why these traits might occur together.
- D
This supports the alternative we worried about. If the sludge contains substantial antibiotics along with the metals, the antibiotics could account for antibiotic resistance. We would have more reason to question which exposure produced the effect, not more reason to single out the metals.
- E
Bacteria elsewhere can have both resistances, but we aren't told how they acquired them or whether they encountered heavy metals. The microbiologist hasn't claimed sewage is the only place resistance can occur. E gives another population with the traits without connecting metal exposure to their cause.
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