Biology Experimental Passage Question

[Jumb0]

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So we are told in the passage that an ulcer causing bacteria produces a highly efficient urease, located on the outer portion of its cell wall, that converts urea to ammonia and carbon dioxide, and that this is the basis of a medical test in which the patient is given a drink of radioactive urea and then tested for the exhalation of radioactive CO2.

The question then asks : "Which of the following experiments would test the hypothesis that urease is necessary for the colonization of the bacteria in the stomach?"

I narrowed it down to these two responses:

A) Exposing ulcer patients to antibodies to urease
B) Exposing uninfected animals to a strain of the bacteria that lacks urease

I picked A, but the answer is B. Here is my issue: In B, we are told that the strain of bacteria "lacks urease". We are not told that it lacks the GENE for urease, therefore we can assume that it might be the case that the strain has the capacity to produce urease but it has simply not expressed the gene for whatever reason. Perhaps it only expresses it once it enters its host. Therefore, if you injected uninfected animals with a strain that "lacks urease", it is liable to turn on the expression of the latent urease gene that it may have, thus causing a false negative. In other words, you would conclude that urease was not necessary for colonization when, in reality, it was.

Answer choice A can be a legitimate test, however. It you successfully get antibodies onto the viral ureases in ulcer patients, you have effectively knocked out the effect of the urease while keeping everything else constant. Therefore, if the patient's ulcers subsided after the administration of these bacterial urease antibodies, then you could say without a doubt that the urease was in fact involved in the colonization of the stomach.

The only issue is that antibodies are made out of protein, which is digestible in the stomach. But if we can make the assumption that answer choice B implied that the strain lacked the gene for urease when it wasn't explicitly stated, then why can't we make the assumption that the antibody of answer choice A was modified or administered in such a way (perhaps as part of a drug cocktail that included pepsin inhibitors) that prevents it from being degraded?

I rest my case.

 

[texan2414]

@Jumb0 which source is this passage from - it seems eerily familiar.

Perhaps if I can relocate it, I'll do (redo) it and then take a crack at this.

 

[RamAttack]

these type of questions you will see on your MCAT because it at its core is asking you for something of a 'control' situation.

I remember this question from the Official Guide book I believe. The reason why A is not the answer is because antibodies just tell you its there but if you expose a person to a bacteria that lacks urease than you see one two outcomes therefore testing the hypothesis. Also if you lack an enzyme you can make the assumption that the the gene is not included.

 

[blackmarble]

it's one of the aamc practice exams, 8 or 9 i think.

when they say the expose the animals to a strain that lacks urease, it is necessarily implied that the gene for urease is knocked out, and won't be expressed. that's the purpose of a knock-out model. i think you're overthinking a bit here.

the reason for A being not as good of an answer is B is because (correct me if I'm wrong), antibodies won't necessarily prevent the action of urease, it's more just to detect the presence of the bacteria itself. in order to actually knock out the action of urease, you would have to give them a vaccine and expose the immune system to an inoculated form of the antigen (which is usually a virus i think, not a bacteria). in this case, the patient could have been exposed previously, and cleared of the bacteria already so it doesn't prove that the bacteria is linked necessarily to ulcers. i'll check the actual passage later and get back if i find differently.