Question: National Lampoon's Christmas Vacation is a film that stars an actor known for playing Dr. Leonard Hofstadter in what CBS sitcom?

Context paragraphs:
Vegas Vacation: Vegas Vacation is a 1997 American comedy film directed by Stephen Kessler.  It is the fourth installment in "National Lampoon"’s "Vacation" film series, and was written by Elisa Bell, based on a story by Bell and Bob Ducsay.  The film stars Chevy Chase, Beverly D'Angelo and Randy Quaid, with Ethan Embry and Marisol Nichols as Griswold children Rusty and Audrey.  The film opened at #4 at the box office and grossed over $36.4 million domestically.  "Vegas Vacation" became the first theatrical "Vacation" film not to carry the National Lampoon label or a screenwriting credit from John Hughes.

National Lampoon's Christmas Vacation 2: National Lampoon's Christmas Vacation 2: Cousin Eddie's Island Adventure is a 2003 American made-for-television comedy film.  It premiered December 20, 2003 on NBC and stars Randy Quaid and Miriam Flynn reprising their roles as Cousin Eddie and Catherine, along with Dana Barron reprising her role as Audrey Griswold.  It is the sequel to the 1989 film, "National Lampoon's Christmas Vacation", although it is more of a spin-off than a direct chapter of the National Lampoon's "Vacation" film series, because Chevy Chase and Beverly D'Angelo did not appear.

Penny (The Big Bang Theory): Penny is a fictional character on the American CBS sitcom "The Big Bang Theory", portrayed by actress Kaley Cuoco.  She is the primary female character in the series, befriending her across-the-hall neighbors Leonard Hofstadter (Johnny Galecki) and Sheldon Cooper (Jim Parsons), two physicists who work at the nearby California Institute of Technology (Caltech).  Penny's lack of advanced education, but outgoing personality and common sense drastically contrast with the personalities of the primary male characters in the series, even though she is considered part of their group.  She is the love interest of Leonard, with whom she maintains a brief romantic relationship during the third season, which is later resumed in the fifth season and culminates in an engagement at the end of the seventh season and a wedding at the start of season 9.  Penny is the only main character of the show whose last name has not been revealed, although she has been occasionally referred to or addressed with the last name Hofstadter since her wedding.

National Lampoon's Christmas Vacation: National Lampoon's Christmas Vacation is a 1989 American Christmas comedy film directed by Jeremiah S. Chechik.  It is the third installment in National Lampoon's "Vacation" film series, and was written by John Hughes, based on his short story in "National Lampoon" magazine, "Christmas '59".  The film stars Chevy Chase, Beverly D'Angelo and Randy Quaid, with Juliette Lewis and Johnny Galecki as the Griswold children Audrey and Rusty, respectively.

William Hickey (actor): William Edward Hickey (September 19, 1927 – June 29, 1997) was an American actor and voice actor.  He is best known for his Academy Award-nominated role as Don Corrado Prizzi in the John Huston film "Prizzi's Honor" (1985), as well as Uncle Lewis in "National Lampoon's Christmas Vacation" (1989) and the voice of Dr. Finklestein in Tim Burton's "The Nightmare Before Christmas" (1993).

National Lampoon, Inc.: National Lampoon, Inc. was incorporated in 1970 to launch the National Lampoon magazine.  They leveraged that success into radio, film , television, and live stage.  The Company developed and produced a number of iconic films, including "National Lampoon's Animal House" and the "National Lampoon Vacation" franchise.

National Lampoon's Vacation (film series): The National Lampoon's Vacation film series is a comedy film series initially based on John Hughes' short story "Vacation '58" that was originally published by "National Lampoon" magazine.  The series is distributed by Warner Bros. and consists of seven films, two of which are not sponsored by "National Lampoon".  In recent years, the series has been the inspiration for various advertising campaigns featuring some of the original cast members.  The series portrays the misadventures of the Griswold family, whose attempts to enjoy vacations and holidays are plagued with continual disasters and strangely embarrassing predicaments.

Holiday Road: "Holiday Road" is a 1983 single written and recorded by Fleetwood Mac guitarist Lindsey Buckingham.  The song was featured in the 1983 film "National Lampoon's Vacation".  The song was also used in the sequels "National Lampoon's European Vacation", "Vegas Vacation" and "Vacation".  While the song only peaked at No. 82 on the "Billboard" Hot 100 chart, it has since become one of Buckingham's best known songs.

Lionel Barrymore: Lionel Barrymore (born Lionel Herbert Blythe; April 28, 1878 – November 15, 1954) was an American actor of stage, screen and radio as well as a film director.  He won an Academy Award for Best Actor for his performance in "A Free Soul" (1931), and remains best known to modern audiences for the role of the villainous Mr. Potter character in Frank Capra's 1946 film "It's a Wonderful Life".  He is also particularly remembered as Ebenezer Scrooge in annual broadcasts of "A Christmas Carol" during his last two decades.  He is also known for playing Dr. Leonard Gillespie in MGM's nine Dr. Kildare films, a role he reprised in a further six films focussing solely on Gillespie and in a radio series entitled "The Story of Dr. Kildare".  He was a member of the theatrical Barrymore family.

Johnny Galecki: John Mark Galecki (born April 30, 1975) is an American actor.  He is known for playing David Healy in the ABC sitcom "Roseanne" from 1992 to 1997 and Dr. Leonard Hofstadter in the CBS sitcom "The Big Bang Theory" since 2007.  Galecki also appeared in the films "National Lampoon's Christmas Vacation" (1989), "Prancer" (1989), "Suicide Kings" (1997), "I Know What You Did Last Summer" (1997), "Bookies" (2003), and "In Time" (2011).

Supporting paragraphs: National Lampoon's Christmas Vacation Johnny Galecki
Answer: The Big Bang Theory