Can someone explain this to me?

The proposal to extend clinical trials, which are routinely used as systematic tests of pharmaceutical innovations, to new surgical procedures should not be implemented. The point is that surgical procedures differ in one important respect from medicinal drugs: a correctly prescribed drug depends for its effectiveness only on the drug's composition, whereas the effectiveness of even the most apporpriate surgical procedure is transparently related to the skills of the surgeon who uses it.

The reasoning in the argument is flawed because the argument

(a) does not consider that new surgical procedures might be found to be intrinsically more harmful than the best treatment previously available.

A is the answer but i dont' quite get it why. Can someone explain?

"consider that new surgical procedures might be found to be intrinsically more harmful than the best treatment previously available." this weaken the argument and it is an alternative explaination.

This is a "Not Weaken" type assumption question.