The White Asparagus Triangulation - S2-E9
[Sheldon points to a computer.]
Sheldon: Tell me what you see here.
Leonard: The blunt instrument that will be the focus of my murder trial?
The White Asparagus Triangulation - S2-E9
Sheldon: Hi Stephanie, I'm sorry I'm late but your companion left me the most undecipherable invitation
Leonard: What invitation?
Sheldon: [From note.] "We're going to the movies" What movie? What theater? What time? If you wanted to make it impossible to find you, you couldn't have done a better job.
Leonard: Oh I could have.
Answer: Because Raj's selective mutism seems to be caused by women that Raj sees (on some sort of conscious or subconscious level) as a potential sexual partner. An attribute that wouldn't apply to either his sister or mother.