> The first, preferred by Fowler, is that the presence of an exception applying to a specific case establishes ("proves") that a general rule exists. A more explicit phrasing might be "the exception that proves the existence of the rule".[1]

* https://en.wikipedia.org/wiki/Exception_that_proves_the_rule

Or perhaps:

> The phrase means that an extreme case is a poor basis for a general law that would cover a wider range of less extreme cases. In other words, a general law is better drafted for the average circumstance as this will be more common.[1]

* https://en.wikipedia.org/wiki/Hard_cases_make_bad_law

Further it can be argued that the fact that a defect is present in the biological system does not invalidate the presence of said system generally. The complete lack of a system could be the determining factor.