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A patient's illness comes to the physician's attention as a clinical problem. The problem that prompts the visit to the physician may be a complaint (e.g. headache) or a complex of symptoms and signs (e.g. fever, rash and sore throat); or the problem may be identified as a finding on physical examination or from the results of diagnostic tests. The physician must solve the problems posed by the patient using information obtained from the history, the physical examination and, when appropriate, diagnostic tests. In the problem-solving process the physician typically develops a problem list that includes differential diagnoses for each of the problems identified. The diagnostic process demands knowledge of disease etiology, pathophysiology and epidemiology and of the patient's gender, ethnicity, environment and prior health status. When the patient is an infant, child or adolescent, the physician must also consider the effects of age, physical growth, developmental stage and family environment. Commonly occurring illnesses will be the first considered, but other, less common disorders may need to be included in the evaluation of various clinical problems.