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Erythema migrans (previously called erythema chronicum migrans) is considered pathognomonic of Lyme borreliosis — a spirochetal infection spread to human beings by bites from infected deer ticks. The ability to make a clinical diagnosis based on the morphology of the rash is critical, because tests frequently produce false negatives (in cases, serology is positive in 50%; histology is nonspecific; culture is difficult and positive in 50%; and borrelial DNA PCR is positive in 36%–88%). A failure to diagnose may delay treatment and allow the infected patient to develop systemic manifestations (cardiac symptoms in 5%–8% of untreated cases; neurologic problems in 15%–20%; arthritis in 30%–60%), but early treatment with appropriate antibiotics p…