I want to know how is the following article an example of case control study?

Question

https://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0231687&type=printable

Answer 1

A case-control study is a study in which you know the outcome and take a retrospective look at the factors. This study is looking for the correlation between HFABP and severity of COVID-19. Our case is the high severity COVID-19 cases, and our assessed risk factor is the level of HFABP detected in these patients. The control is the population of patients diagnosed with COVID-19.

I hope this clears it up for you, and please feel free to reach out with other questions.