#22 - Microbiologist: Because heavy metals are normally

[Administrator]

Complete Question Explanation

Strengthen—CE. The correct answer choice is (B)

The microbiologist argues that since bacteria that live in sewage sludge that contains heavy metals are resistant to both heavy metal poisoning as well as to antibiotics. The microbiologist then concludes that the exposure to heavy metals has caused the resistance to antibiotics. The causal indicator word is “promoted,” which the author uses to indicate that the exposure caused the resistance. We can diagram the relationship as follows:

• Cause Effect
  
  Exposure to heavy metals Resistance to antibiotics

The author does not provide much information to support the existence of a causal relationship between exposure and the resistance. It does support a correlation; that the two occur together. To strengthen this argument, we need to strengthen the causal relationship by (1) reducing the likelihood of an alternate cause, (2) reducing the likelihood that the cause occurs without the effect, (3) reducing the likelihood that the effect occurs without the cause, or (4) reducing the likelihood that the cause and effect are reversed.

Answer choice (A): Though this answer choice looks tempting, as it seems to connect heavy metal resistance to antibiotic resistance, it does not connect exposure to heavy metals with resistance to antibiotics. The idea of exposure is central to the causal relationship, and since this answer choice just connects resistance to heavy metals to resistance to antibiotics, it does not strengthen the causal relationship in the stimulus.

Answer choice (B): This is the correct answer choice. It states that without exposure to heavy metals, bacteria generally are not resistance to heavy metals or antibiotics. It strengthens the argument by saying that when the cause does not occur, the effect also does not occur. Thus, by reducing the likelihood of the effect occurring without the cause, it is correct.

Answer choice (C): This answer choice does not support the causal relationship. Remember that causal relationships only go in one direction, with the cause occurring prior to the effect. This answer choice suggests that the antibiotic resistance itself caused heavy metal poison resistance, which is not the causal relationship concluded by the microbiologist.

Answer choice (D): Even if the sludge contains antibiotics, it does not support that the resistance to antibiotics came from the exposure to the heavy metals. In fact, if the sludge contained both heavy metals and antibiotics, it would be more difficult to determine if the resistance came from exposure to heavy metals, antibiotics, or some combination.

Answer choice (E): Bacteria can be exposed to heavy metal other than in sludge. The causal relationship is not specifically limited to situations where the exposure to heavy metals occurred in sludge. This answer choice actually has no impact on the argument, because we do not know if the bacteria were exposed to heavy metal or not.

[kcho10]

Hi,
Can someone explain why (A) is incorrect? It seems like it suggests that if the effect does not occur, the cause does not occur. Does that not strengthen the argument?
Thank you!

[Nikki Siclunov]

Hi kcho10,

This is a tricky argument, so let's break it down:

The author observes that bacteria that live in sewage sludge containing heavy metals are resistant to both heavy metal poisoning and antibiotics. From this correlation, she concludes that the exposure to heavy metals has promoted the resistance to antibiotics:

Exposure to heavy metals (cause) Resistance to antibiotics (effect)

From the correlation between the two types of resistance, it appears possible that exposure to heavy metals could cause resistance to antibiotics, but who knows? Maybe exposure to antibiotics causes resistance to heavy metals. And what if there is a third, independent cause that promotes resistance to both? To strengthen this argument, we need to strengthen the causal relationship by (1) reducing the likelihood of an alternate cause, (2) reducing the likelihood that the cause occurs without the effect, (3) reducing the likelihood that the effect occurs without the cause, or (4) reducing the likelihood that the cause and effect are reversed.

You're correct in noting that answer choice (A) is awfully attractive. However, answer choice (A) does not provide an example in which neither the cause nor the effect occurs. Why? Because the cause in our argument is exposure to heavy metals, not resistance to heavy metals. Granted, all bacteria that develop resistance to something must have been exposed to it, but not all bacteria that were exposed to something necessarily become resistant to it. All we know from answer choice (A) is that resistance to antibiotics and resistance to heavy metal poisoning are correlated. But we knew that already - it was the premise of the argument. There is no need to strengthen the correlation any further: that would have no bearing on the conclusion of the argument. Our job is to support the view that exposure to heavy metals causes resistance to antibiotics.

That's why (B) works so well: it shows that when the cause doesn't occur, the correlation between the two types of resistance does not even exist.

Does that make sense? Let me know.

Thanks!

[lsatnoobie]

Is A wrong b/c it is (almost) the contrapositive of the casual conclusion "Exposure to heavy metals —> Antibiotic Resistance"

and A is saying /Antibiotic Resistance –> /Exposure to heavy metals

So A isn’t doing much to strengthen?

[Brook Miscoski]

lsatnoobie,

Read the concluding sentence of the stimulus carefully.

The stimulus presents the causal argument that exposure to heavy metals causes antibiotic resistance.

Therefore (A), which is not about the proposed cause (heavy metal exposure), is not relevant to evaluating the causal argument. That is why you should eliminate (A).

Remember, showing that when the cause is absent the effect is absent is a good way of strengthening a causal argument, not a mistaken negation. That is why it is usually important not to mix and match conditional and causal reasoning. In this case, (B), the correct answer, states that when the cause (heavy metals) is absent, the effect (antibiotic resistance) also tends to be absent.

[ericau02]

I understand the correct answer choice. I originally picked a and during my blind review I realized it was actually ac B. But I am confused as to what the error is in ac E. Because I am doing blind review I am trying to understand why certain ac are wrong.

My original notation was that ac E was incorrect because it was showing the effect resistance to both heavy metals and antibiotics without the cause, by stating "... kinds of bacteria that do not live in sewage sludge"

is this a correct analysis or is it incorrect for a different reason?

[Rachael Wilkenfeld]

Hi Erica,

It's great that you are going over correct and incorrect answers. It's really important to understand WHY things are right or wrong during practice so that you can easily eliminate incorrect answers.

Our causal relationship here is that exposure to heavy metals causes antibiotic resistance. To strengthen that, we need to strengthen the relationship between that cause and that effect. Answer choice (E) seems to say all the right things, but it's missing a critical component. It says that many kinds of bacteria are resistant to both heavy metal poisoning and antibiotics, but it doesn't give us any information about whether those bacteria have themselves been exposed to heavy metals. Because of that, we can't link it up to our causal relationship in the stimulus. As the answer choice is written, it has no impact on the stimulus.

Hope that helps!
Rachael

[ericau02]

Hi Erica,

It's great that you are going over correct and incorrect answers. It's really important to understand WHY things are right or wrong during practice so that you can easily eliminate incorrect answers.

Our causal relationship here is that exposure to heavy metals causes antibiotic resistance. To strengthen that, we need to strengthen the relationship between that cause and that effect. Answer choice (E) seems to say all the right things, but it's missing a critical component. It says that many kinds of bacteria are resistant to both heavy metal poisoning and antibiotics, but it doesn't give us any information about whether those bacteria have themselves been exposed to heavy metals. Because of that, we can't link it up to our causal relationship in the stimulus. As the answer choice is written, it has no impact on the stimulus.

Hope that helps!
Rachael

So was my analysis correct to say that it is stating the effect which is the resistance of both the heavy metal poisoning and antibiotics but did not state the exposure of heavy metal sludge?

[James Finch]

Hi Erica,

There are two casual relationships occurring in the stimulus: heavy metal concentration in sewage sludge leads to heavy metal resistance in bacteria, which is undisputed, and in turn this heavy metal resistance supposedly leads to antibiotic resistance. We need to bolster the heavy metal resistance antibiotic resistance causation, but in order to do that, we would need a control group of bacteria that live in non-sludge water that is also not contaminated by heavy metals (as the stimulus leaves open the possibility that there is water that is contaminated by heavy metals but not sewage sludge).

(B) gives us this setup, which bolsters the correlation between the cause and effect, as the control group usually (generally) lacks both antibiotic and heavy metal resistances. So the causal chain would remain intact:

Heavy Metal Contamination (HMC) Heavy Metal Resistance (HMR) Antibiotic Resistance (AR)

because HMC HMR AR

(E), however, doesn't do this, and the word "many,"-which tells us only that there are some bacteria for which this is true, but not the proportion, unlike "generally," a synonym for "most"-although not determinative on its own, should be used as a clue that this is a suspect answer choice. One issue with (E) is that it only tells us about non-sewage sludge water, not water that lacks heavy metal contamination. This means that the evidence it presents is largely useless, because the important underlying causal element is the heavy metal concentration, not the sewage sludge itself. But the most damning issue is that it is effectively an opposite answer, in that the way it is presented could only hurt the likelihood that the stimulus's conclusion is correct, as it could only show HMC HMR and AR, as you stated, which would weaken the arguement. And it doesn't even necessarily do that.

Hope this clears things up!

[whardy21]

I chose B for this question. The reason I chose B is because if bacteria that live in sewage sludge without the heavy metals show they are not resistant to heavy metal poisoning or nor antibiotics, it shows that without the heavy metals they aren't resistant to either. If bacteria live in heavy metal sewage show resistant to antibiotics, which states that in the premises of the stimulus. Thats how I looked at it.