Erythema chronicum migrans is the pathognomonic skin lesion of Lyme disease, caused by the spirochete Borrelia burgdorferi transmitted through tick bites. It appears as a slowly expanding, erythematous, and often annular rash at the site of the tick bite, typically within 3 to 30 days. This lesion is not seen in syphilis, measles, or leprosy. Recognition of erythema chronicum migrans is critical for early diagnosis and treatment of Lyme disease to prevent systemic complications.

Reference: Harrison's Principles of Internal Medicine, Fauci et al., 20th Edition.