Mice that have been given morphine are very likely to develop blood poisoning because bacteria that normally reside in the intestine typically respond to morphine by migrating into the bloodstream. However, when mice are given both morphine and the new drug naltrexone, blood poisoning is much less frequent, although it does still occur. These results provide support for researchers' prediction that naltrexone will turn out to be toxic to certain types of bacteria. Which of the following, if discovered to be true, would most seriously weaken the support for the researchers' prediction? ['After being administered to mice, naltrexone does not pass from the bloodstream into the intestine.', 'Naltrexone inhibits morphine from triggering the migration of intestinal bacteria into the bloodstream.', 'Mice that have been given naltrexone but not morphine have no greater risk of developing blood poisoning than do mice that have not been given either substance.', 'The increased risk of blood poisoning is not the only harmful effect on mice of being given morphine.']