This patient's skin findings are most likely caused by vasculitis of the cutaneous blood vessels. Inflammation of the blood vessels may lead to vascular injury with hemorrhage into the skin. The resulting clinical lesions are described by their size. Petechiae are pinpoint macules (<3 mm in diameter), which are seen most commonly in platelet disorders, while purpura causes more widespread areas of involvement (up to 1 cm in diameter). Ecchymoses are confluent purpuric areas larger than 1 cm in diameter. Vasculitic lesions are nonblanchable, and when vasculitis involves the small cutaneous vessels, they may be palpable, resulting in the characteristic diagnostic finding in cutaneous vasculitis of palpable purpura. Small vessel vasculitis may be idiopathic but may also occur in association with systemic diseases, including infection, sepsis, a more diffuse vasculitic process or autoimmune disorder, medication reactions, or rarely malignancy. Because vasculitic skin findings may reflect a significant underlying disease process, a thorough evaluation is required, particularly in this patient with fever, joint and muscle pain, and possible focal weakness.

Erythema multiforme (EM) is believed to be a cell-mediated immune process that occurs in response to a specific trigger, which is most commonly infectious (and particularly associated with herpes simplex virus infection). EM often manifests as a three-ring target on the palms, and the center of the target can have a dark red or purple appearance; the dusky center may become necrotic and can form a discrete blister or eschar. Lesions range in size from several millimeters to several centimeters. Few to hundreds of lesions develop within several days and are most commonly located on the extensor surfaces of the extremities, particularly the hands and feet. EM, however, does not cause widespread purpura.

Lichen planus (LP) is an inflammatory skin disorder of unclear etiology that can affect the skin, mucous membranes, scalp, and/or nails. The hallmark lesion of LP is the pruritic, purple, polygonal papule; these papules are usually distributed symmetrically on the wrists, flexural aspects of the arms and legs, lower back, and genitals. A reticulated network of gray-white lines (Wickham striae) may be visible on the surface of the papules. LP does not cause purpura.

Traumatic ecchymosis can cause extensive purpura, especially in older patients who have age-related thinning of the skin and vascular structures and are more likely to take antiplatelet or anticoagulant medications; however, traumatic bruising is more often localized and may have a pattern that suggests the external cause of injury.