In this directed case study students follow a nurse practitioner and work with a diagnostics team to determine what is wrong with Tristan, an infant who comes to the clinic with multiple bruises. Students are given background and patient history, and are then given results of various blood tests ordered by the diagnostics team. The exercise emphasizes the physiological process of coagulation and the importance of various clotting factors, especially factor VIII. Students will be introduced to results from several blood tests, including: complete blood count, partial thromboplastin time, prothrombin time, metabolic panel, and factor VIII assay. The patient is ultimately diagnosed with severe hemophilia A and the case then introduces students to this disorder, the genetic determinants, the incidence, and ways to manage the disease. The data in this case are real and the story represents the medical history of an actual patient. Originally developed for pre-nursing students, this activity would also be suitable for majors in physiology or pre-medical students; it could also be used in an introductory genetics or biology course.