Question: Which voice actress first appears in the South Park episode Cartman gets an anal probe?

Context paragraphs:
Cartman's Mom Is a Dirty Slut: "Cartman's Mom Is a Dirty Slut" is the first season finale of the American animated television series "South Park".  It originally aired on Comedy Central in the United States on February 25, 1998.  The episode is the highest viewed episode in the entire "South Park" series, with 6.4 million views.  It is part one of a two-episode story arc, which concluded with "Cartman's Mom Is Still a Dirty Slut".  The episode follows Eric Cartman, one of the show's child protagonists, becoming curious about the identity of his father.  He discovers that his father is most likely a man his mother had sexual intercourse with during an annual party called "The Drunken Barn Dance".  Meanwhile, his friends Stan, Kyle and Kenny participate on "America's Stupidest Home Videos", after filming Cartman playing in his yard with plush toys.

Mr. Garrison: Herbert Garrison, formerly known as Janet Garrison, is a fictional character in the American animated television series "South Park", He is voiced by Trey Parker and first appeared on television on August 13, 1997 in "South Park"' s pilot episode "Cartman Gets an Anal Probe".

Wendy Testaburger: Wendy Testaburger is a fictional character in the American animated series "South Park".  The series' most prominent female character, she is best known for her on-again, off-again relationship with her boyfriend, Stan Marsh, and being more intelligent and mature than most children her age, which is utilized by her activism and feminism.  Wendy debuted as a nameless background character in Trey Parker and Matt Stone's 1995 college short film, "The Spirit of Christmas", and made her first appearance on television when "South Park" initially premiered on Comedy Central on August 13, 1997 with the episode "Cartman Gets an Anal Probe".  She is currently voiced by April Stewart.  Prior, she had been voiced by several different voice actors throughout the show's run: Mary Kay Bergman, Eliza Schneider, and Mona Marshall.

Major Boobage: "Major Boobage" is the third episode in season 12 of the American animated television series "South Park".  The 170th episode of the series overall, it originally aired on Comedy Central in the United States on March 26, 2008.  The episode was co-written by series creators Trey Parker and Matt Stone; it was the first South Park episode since 2000 not solely written by Parker, ending a streak of 120 consecutive straight solo episodes.  The previous episode co-written by Stone was "The Tooth Fairy's Tats 2000".

Medicinal Fried Chicken: "Medicinal Fried Chicken" is the third episode of the fourteenth season of the American animated television series "South Park", and the 198th episode of the series overall.  It originally aired on Comedy Central in the United States on March 31, 2010.  In the episode, the South Park KFC is replaced by a medical marijuana dispensary, and Cartman gets involved in black market selling the KFC chicken.  Meanwhile, Randy Marsh gets a medical referral for marijuana by giving himself testicular cancer, which makes his testicles grow to grotesquely huge proportions.

Eric Cartman: Eric Theodore Cartman, often referred to as just Cartman, is a main character in the animated television series "South Park", created by Matt Stone and Trey Parker, and voiced by Trey Parker.  Cartman, generally referred to by his surname, is one of four central characters in "South Park", along with his friends Stan Marsh, Kyle Broflovski, and Kenny McCormick.  Cartman first appeared, originally named Kenny, in prototypical form in a 1992 animated short "Jesus vs. Frosty", and a 1995 animated short "Jesus vs. Santa", and first appeared on television in the pilot episode of "South Park", "Cartman Gets an Anal Probe", on August 13, 1997.

Cartman's Mom Is Still a Dirty Slut: "Cartman's Mom Is Still a Dirty Slut" is the second episode in the second season of the American animated television series "South Park".  The 15th episode of the series overall, it premiered on Comedy Central in the United States on April 22, 1998.  The episode concludes the storyline of the season one finale "Cartman's Mom Is a Dirty Slut"; Mephesto is suddenly shot, just as he is about to reveal the identity of Cartman's father.  The four boys and Chef rush him to Hell's Pass Hospital while the town of South Park experiences a massive blizzard.

Cancelled (South Park): "Cancelled" (also known as Cartman Gets an Anal Probe Redux) is the first episode of the seventh season of the American animated television series "South Park", and the 97th episode of the series overall.  It first aired on Comedy Central March 19, 2003.

Cartman Gets an Anal Probe: "Cartman Gets an Anal Probe" is the first episode of the American animated television series "South Park".  It first aired on Comedy Central in the United States on August 13, 1997.  The episode introduces child protagonists Eric Cartman, Kyle Broflovski, Stan Marsh and Kenny McCormick, who attempt to rescue Kyle's younger brother Ike from being abducted by aliens.

The Biggest Douche in the Universe: "The Biggest Douche in the Universe" is the 15th episode of the sixth season and the 94th overall episode of the Comedy Central animated series "South Park".  It was first broadcast on November 27, 2002 and was the last in a mini-arc depicting Cartman being occasionally possessed by Kenny.  The episode is centered on Liane Cartman and Chef attempting to exorcise Kenny's soul while Stan deals with trying to debunk self-proclaimed psychic John Edward, whom the entire crew of South Park Studios agreed to be "the biggest douche in the universe."  The episode is rated TV-MA in the United States, except on syndicated broadcasts, where the episode is rated TV-14, instead.

Supporting paragraphs: Wendy Testaburger Cartman Gets an Anal Probe
Answer: April Stewart.