Question

A doctor prescribed Patient X the antibiotic tetracycline to treat an infection involving the bacteria Streptococcus agalactiae. The antibiotic worked, and Patient X regained his health. Several weeks later, Patient K, a family member of Patient X, took tetracycline to treat an infection involving the same bacteria. This time the antibiotic did not work, and Patient K had to seek out another treatment.What statement best explains why tetracycline did not effectively treat Patient K's S. agalactiae infection?A. The generation of S. agalactiae responsible for the first infection were smaller in number and comprised of older and weaker bacteria.B. The generation of S. agalactiae responsible for the second infection descended from bacteria that were selected for their resistance to tetracycline.C. Patient K waited too long to begin treatment, so the bacterial infection had progressed to the point that it was untreatable with an antibiotic.D. Patient K had a rare genetic condition, which causes tetracycline to be ineffective in treating bacterial infections.

Answer 1

Even if the generation of bacteria was smaller, older or if the infection was going on for a long time, the antibiotics should be able to treat both infections, as its action does not depend on the amount or age of the bacteria. Therefore, A and C are both incorrect.

Rare genetic conditions usually appears in other members of the family as well. And there is no indication of the genetic condition. So D is wrong too.

The most reasonable explanation would be that the bacteria developed the ability to kill the drugs designed to kill them. As they are from the same family, it is possible that K was infected with the offspring of the bacterias in X. Therefore, the correct answer is B. The generation of S. agalactiae responsible for the second infection descended from bacteria that were selected for their resistance to tetracycline.