This is erythema nodosum, a type of panniculitis manifested by a central tender nodule with surrounding erythema. The lesions may reach several centimeters in size over weeks to months and then fade. Lesions are most common on anterior leg and thigh. In time, they will become purple, then flat and brown, then fade out. In some cases an underlying systemic inflammatory condtion, such as a granulomatous disease, is present, but in many cases this condition is idiopathic.

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