Answer the following question: Given the following passage  "The University of Oklahoma has several institutions of higher learning in the city and metropolitan area, with OU Medicine and the University of Oklahoma Health Sciences Center campuses located east of downtown in the Oklahoma Health Center district, and the main campus located to the south in the suburb of Norman. The OU Medicine hosting the state's only Level-One trauma center. OU Health Sciences Center is one of the nation's largest independent medical centers, employing more than 12,000 people. OU is one of only four major universities in the nation to operate six medical schools.[clarification needed]",  answer the following question. Note that the answer is present within the text.  Question: There are how many universities in charge of operating medical schools?
Answer:
four