Dr Zabel (Scott Grimes) mentions that he is old friends with Frank (William H. Macy) in the hospital. They were both doctors on ER (1994) which was also set in Chicago.
In the scene where Lip is studying in the hospital, he has a black satchel/messenger bag with a skull and crossbones patch that reads "THE ALLEY CHICAGO". The Alley is a long-time record and clothing store on Belmont Avenue in Chicago that has catered to the counter culture, alternative and punk rock styles. The patch shown in the episode is sewn inside their jackets where the sizing is listed.