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An investigator wishes to know whether PED patients seen with bloody diarrhea are prescribed antibiotics in the PED are more likely to develop hemolytic uremic syndrome (HUS). Over a 10-year period, several cases of patients with HUS diagnosed after a PED visit for diarrhea are identified, and for each of these patients, 3 other patients seen in the PED for diarrhea that did not develop HUS matched for age, gender, duration of illness, and stool culture findings are identified. The two groups of subjects are compared regarding whether they received PED antibiotics or not.

What is the most appropriate description for this type of study?