A student recently asked, "If that is the logic behind the answer, couldn't just "tr1 and tr2" be the answer? Why is it specifically 2 and 3 that must be chosen?"
In this question, we are told that there are six distinct serotypes (or strains) of a given virus. In the table, we are given three antigens for each of the six serotypes. Each is labeled either "pg," which indicates an antigen that is not crucial, or tr, which indicates an antigen that is crucial. In other words, without a "pg" antigen, the virus could still propagate, but losing a "tr" would mean the virus could not.
The question asks for the minimum that a drug must block in order to be an effective treatment. To be effective, it must block a "tr" antigen for each of the serotypes. Each row (one for each serotype) contains either tr2 or tr3, so a drug that blocks these two would render all six serotypes unable to propagate. Option D would also render all six serotypes unable to propagate, but it is not the minimum requirement (since blocking tr1 would essentially be overkill).
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