Medicine: PreTest Self-Assessment and Review, 13th Edition, is intended to provide medical students, as well as house officers and physicians, with a convenient tool for assessing and improving their knowledge of medicine. The 500 questions in this book are similar in format and complexity to those included in the Step 2 of the United States Medical Licensing Examination (USMLE). They may also be a useful study tool for Step 3 and for National Board of Medical Examiners (NBME) Medical student exam for the internnal medicine clerkship. To simulate the time constraints imposed by the qualifing examinations for which this book is intended as a practice guide, the student or physician should allot about one minute for each question. After answering all questions in a chapter, as much time as necessary should be spent in reviewing the explanations for each question at the end of the chapter. Attention should be given to all explanations, even if the examinee answered the question correctly. Those seeking more information on a subject should refer to the reference materials listed or to other standard texts in medicine.