The Recombination Hypothesis - S5-E13
Factual error: Leonard lies to Sheldon about his being over at Penny's at 3am by saying he went across the street to use the gas station bathroom. Sheldon refers to it as "The Colorado Boulevard Chevron." The address of their apartment building is 2311 N. Los Robles. Colorado Boulevard is five miles away. (Yes, it's a fantasy sequence, but even in Leonard's head, Sheldon's not likely to make a mistake like that). (00:15:45)
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.