A key part of the physical examination is to identify a primary lesion (i.e., a new lesion) as well as any older lesions, which determine the morphologic progression of the rash. It is also important to recognize that patients may manipulate the skin in a way that alters the appearance of the rash, either through scratching or application of topical medications or other treatments. The examination should begin with an assessment of whether the patient looks well or has a toxic appearance. All areas of the skin should be examined systematically, usually beginning with the head and neck and proceeding to the trunk, buttocks and genitalia, and extremities, including the palms and soles. Examination of the mucous membranes (conjunctivae, lips and orophaynx, and urethral meatus) is important. Potentially severe drug hypersensitivity reactions such as Stevens–Johnson syndrome are differentiated from more benign drug hypersensitivity reactions on the basis of mucous membrane involvement. The examination should also include an assessment of the presence or absence of regional or systemic lymphadenopathy, which may be associated with certain infections or inflammatory conditions, and an assessment of the musculoskeletal examination, as arthralgis and arthritis are seen with several systemic inflammatory diseases and with some infections, such as parvoirus B19 infection. Finally, hair and nails should be examined, as abnormalities may indicate signs of systemic disease, such as systemic lupus erythematosus.