In response to viral infection, the immune systems of mice typically

Start with the scientists' hypothesis. We are told that mice infected with a herpesvirus generally develop keratitis. The hypothesis is that "these cases of keratitis (those in mice infected with herpesvirus) are caused by antibodies to herpesvirus".

Now that we've identified the hypothesis, let's look at how the scientists arrived at this hypothesis:

• When mice are infected by a virus, their immune systems produce antibodies to fight the infection.
• These antibodies are supposed to destroy the virus by binding to proteins on its surface.
• So if a mouse is infected with herpesvirus, we would expect the mouse's immune system to produce antibodies that would bind to proteins on the virus's surface. Great!
• Unfortunately, mice with herpesvirus generally develop keratitis, a degenerative disease affecting part of the eye. Interesting... why does that happen?
• Well, proteins on the surface of cells in this part of the eye closely resemble those on the herpesvirus surface. Uh oh... maybe the antibodies that are supposed to bind to the herpesvirus proteins are ALSO binding to proteins in the eye, since those proteins closely resemble those of the virus.
• So the antibodies are supposed to be attacking the virus, but, if the hypothesis is correct, then those antibodies are also attacking cells in the eye.

This hypothesis seems to make sense, but we need an answer choice that gives it the greatest additional support:


In order for this statement to support the hypothesis, we would need additional information. What happens when mice are infected with those other types of virus? Do the organs with similar proteins suffer any damage? If so, then the hypothesis would be supported. But without further evidence, choice (A) doesn't help. Eliminate (A).


These poor mice won't be able to fight the herpesvirus. If these mice also did NOT develop keratitis once infected with the herpesvirus, then we would have some solid support for the hypothesis. Unfortunately, we are not given such information.

All we know is that the mice who can't fight the herpesvirus contract the virus at about the same rate as other mice. We aren't interested in herpesvirus contraction rates. We are interested in the cause of the keratitis that generally develops in the infected mice. Eliminate (B).


This statement probably weakens the hypothesis. The scientists argue that the keratitis is caused by the antibodies. If these mice produce the same levels of antibodies and the antibodies cause the keratitis, then why wouldn't those mice also develop keratitis? This suggests that the antibodies aren't actually causing the keratitis.

If infected mice that do not develop keratitis did NOT produce as many antibodies as infected mice that do develop keratitis, then we would have support for the hypothesis. But this is not what statement (C) says, so eliminate (C).


Ah ha! This is what we were looking for in choice (B). The mice are unable to form antibodies in response to herpes infections. These poor mice cannot fight the virus, but they also do not develop keratitis. No antibodies, no keratitis. This certainly supports the scientists' hypothesis, so hang on to (D).


Notice that the hypothesis specifically refers to "THESE CASES of keratitis." Which cases? Those that develop in mice infected with herpesvirus. So the scientists are not trying to explain causes of ALL cases of keratitis. Instead, they are only trying to explain the cases that develop in mice infected with herpesvirus.

Choice (E) does suggest that there are other ways to develop keratitis (besides the way proposed by the scientists). If anything, this slightly weakens the hypothesis by implying that something else (besides the antibodies to herpesvirus) could be causing these cases of keratitis. At best, choice (E) is irrelevant. At worst, it weakens the hypothesis, so eliminate (E).

Choice (D) is the best answer.