Not only is the number 73 important marking the year actor Jim Parsons was born, this is also the 73rd episode of the show.
The whiteboard behind Sheldon as he tries to do a differential diagnosis on Amy's symptoms shows a schematic for the metabolism of glucose.
The is a rare instance where the episode title is actually stated directly (including the word "The"), by Sheldon during the differential diagnosis.
While Sheldon and Penny are talking in the laundry room, there's a sign behind Penny on the wall that reads: "Please Keep Your Clothes On While Doing Laundry"
The Irish bar where Sheldon and Amy go and meet Zack might very well be The Lucky Baldwin, the Irish pub mentioned in The Irish Pub Formulation (2010).