Not only is the number 73 important marking the year actor Jim Parsons was born, this is also the 73rd episode of the show.
During one of the wrestling scenes Howard (Simon Helberg) warns Raj that he took a karate class as a child. According to Helberg's IMDB biography "his main interest as a young boy was karate."
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"