Establishing a medical diagnosis serves two utilitarian purposes: providing information necessary to initiate treatment and communicating information regarding prognosis. A nosology or diagnostic nomenclature (i.e., a classification of diagnoses) provides further utility by establishing a foundation for clinical research. In his book, Wulff outlined four types of diagnoses: (1) symptomatic or pseudoanatomic diagnoses (e.g., chronic headache, persistent diarrhea, or irritable bowel); (2) syndromes; (3) anatomic diagnoses; and (4) causal diagnoses. By definition, syndromes have no means of being validated by measures external to the constructs themselves. Often, specific syndromes reflect diverse origins, and conversely, specific etiologies may cause multiple syndromes (e.g., syphilis, human immunodeficiency virus, and diabetes).