What piece did the string quartet play in Hamburger Postulate episode of The Big Bang Theory?

The string quartet played the first movement of Beethoven's String Quartet No. 14 in C sharp minor, Op. 131 in the "Hamburger Postulate" episode of The Big Bang Theory.

This piece was chosen for its dramatic and emotional qualities, fitting the tension in the episode as Sheldon faces his fear of eating hamburgers.