Admission into medical school may not technically require completion of a previous degree; however, applicants are usually required to complete at least 3 years of "pre-med" courses at the university level because in the US medical degrees are classified as Second entry degrees. Once enrolled in a medical school the four years progressive study is divided into two roughly equal components: pre-clinical (consisting of didactic courses in the basic sciences) and clinical (clerkships consisting of rotations through different wards of a teaching hospital). The degree granted at the conclusion of these four years of study is Doctor of Medicine (M.D.) or, less commonly, Doctor of Osteopathic Medicine (D.O.) depending on the medical school; both degrees allow the holder to practice medicine after completing an accredited residency program.[1][2]