If anything, this weakens the doctors' position. If treatment for high bilirubin levels reduces the risk of tetanus and has no negative side effects, then it's not clear why this treatment should be withheld, which is what the second group of doctors is arguing for.

This weakens the doctors’ argument. The doctors base their argument on the premise that the brain's own natural defenses normally permit bilirubin from entering. But this answer choice tells us that there are known cases where the normal defenses don't work as well, and babies run the risk of tetanus.

This is irrelevant. The disagreement is over whether or not overall bilirubin levels should be lowered. Knowing that bilirubin appears elsewhere besides the blood doesn't clearly affect this question.

This strengthens the argument. It tells us why, even though there is a potential risk of tetanus from bilirubin, these doctors think bilirubin should not be lowered. Bilirubin carries a distinct benefit for newborns, and tetanus is not a risk in normal situations. Thus, bilirubin levels shouldn’t be lowered.

We're not interested in what this first group of doctors think. We’re interested in the second group of doctors, and why they think bilirubin levels shouldn’t be lowered in newborns.