Pratt, the sister of notorious "Hills" villain Spencer Pratt and the now step-sister to Heidi Montag, was cuffed by cops Oct. 18 in Hollywood. She pleaded not guilty to misdemeanor DUI Friday, Usmagazine.com reported.
Pratt, 23, will spend 30 days in a residential facility and is facing up to six months in jail plus a $1,000 fine for the offense.
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The fashion student is featured on the show alongside Spencer Pratt and Montag as well as friends Audrina Patridge and Lauren "Lo" Bosworth.