Clinicians are trained to employ the differential diagnostic method using a wide range of diagnostic methods and tests to eliminate diagnostic possibilities and narrow the diagnostic list. The history and physical exam are the most important first steps in the differential diagnosis. In diseases of either systemic or organ-specific damage owing to oxidative stress, diagnostic testing usually depends on biomarker measurements and imaging techniques. When the amount of biomarker information increases to the point of being hard to conceptualize, the data are presented as an image. Endoscopy has transformed diagnosis and surgery, with an endoscopic instrument available for almost every organ system, even the eye. Radioactive isotopes are used in laboratory investigation in a wide range of applications to label molecules in cellular structures and biochemical reactions. Single-photon emission computed tomography (SPECT) bone scan produces a 3D image, allowing better localization of a defect.