Yes, there is a typo in the part above question 2c -- you are correct that it should read:
"...the cases and controls in this study were adolescent ED patients with symptoms suggestive of PE ..."
As for what is meant by the "question stem" -- that would refer to the general description of how the study used the d-dimer and CTPA tests. I would suggest reading from "For this reason, ... " through the description of the Sharaf study.