Allow me to present a common clinical scenario. The patient presents with right lower quadrant abdominal pain. Appendicitis is in the differential diagnosis. After an appropriately thorough H&P you have ascertained that it is not gastro/UTI/strep/pneumonia/constipation/porphyria/pregnancy etc., etc,. and obtain an ultrasound which shows a normal appendix but reveals the presence on enlarged mesenteric lymph nodes.