The detail from the text that most effectively shows the author's choice of compare/contrast text structure in helping understand Dr. Semmelweis’s impact is:
“The mortality rate in the First Clinic dropped from 7.82% for the first half of 1847 to 3.04% for the second half.”
This statement highlights the contrast between the mortality rates before and after Semmelweis’s intervention, clearly illustrating his impact on the reduction of deaths due to childbed fever and emphasizing the effectiveness of his hand-washing protocol.