Alexander "Xand" van Hoogenhouck-Tulleken and Christoffer "Chris" van Hoogenhouck-Tulleken, known as "Dr. Chris" and "Dr. Xand" are British doctors, television presenters and identical twin brothers. They are best known for presenting the children's series Operation Ouch!. Chris has become well known for his two-part television special for BBC One entitled The Doctor Who Gave Up Drugs. Xand has also presented shows without his brother.

Operation Ouch! is a British comedy children's television series on the human body, showing what happens in A&E, what doctors sometimes have problems with and experiments. The first series of Operation Ouch! aired on CBBC in October 2012 and ABC Australia in 2013. The show is hosted by twin brothers and doctors Chris and Xand van Tulleken and in 2019 a new doctor, Ronx İkharia, was introduced. Series 9 first aired on CBBC starting May 2020 for one episode, and then resumed on 13 January 2021.