In The Big Bang Theory Season 4, Episode 10, Sheldon and Amy's relationship faces its first biological "crisis." After Amy experiences a physical reaction to Penny's ex-boyfriend Zack, she and Sheldon attempt to diagnose her symptoms with rigorous scientific logic. The "Diagnosis" Breakdown
Fans often point out that Sheldon and Amy eat lunch in the lab while Amy is slicing brain tissue—a major violation of safety protocols. [S4E10] The Alien Parasite Hypothesis
Sheldon proposes several possible causes for Amy’s elevated heart rate, sweaty palms, and "localized vascular throbbing": Ruled out via blood work. In The Big Bang Theory Season 4, Episode
Dismissed by Amy (despite Sheldon's "testy bark of an old biddy" comment). [S4E10] The Alien Parasite Hypothesis
After rejecting Zack's lack of intellect, Amy adopts the Vulcan practice of Kolinahr to purge her emotions.