Clinical trainees and practitioners can enhance their differential diagnosis knowledge using this completely free app (no subscription necessary). The VanderbiltDDx Quiz App can generate tens of thousands of questions at user-specified levels (easy, moderate, difficult) based on its internal cache of over 600 evidence-based internal medicine disease descriptions. Users can also select question styles and subspecialty focus topics for questions. Styles include: factors predisposing to a disease; the relative predictive values of findings for diseases, the relative frequencies of specific findings in diseases, how often diseases co-occur, and more. The brevity of each multiple-choice question allows users to learn over brief or prolonged sessions. With this app, user can also track their accuracy over time, review past questions, and link out to relevant bibliographical resources.