A 34-year-old man returns from a hiking trip in the northeastern United States and presents with a single enlarging erythematous lesion on his right thigh 9 days after removing an engorged tick from the area. The lesion began as a small red macule and has expanded centrifugally over one week to 12 cm diameter. The structure marked **A** in the diagram shows the characteristic appearance of this lesion. He also reports low-grade fever, fatigue, headache, myalgias, and regional lymphadenopathy. What is the most likely diagnosis?
A. Granuloma annulare with superimposed cellulitis
B. Secondary syphilis with annular lesions and systemic symptoms
C. Erythema multiforme triggered by recent viral infection
D. Early localized Lyme disease caused by Borrelia burgdorferi transmitted by Ixodes tick
Explanation
Why Early localized Lyme disease caused by Borrelia burgdorferi transmitted by Ixodes tick is right
The structure marked A — the expanding annular erythema with centrifugal spread, brighter advancing red border, and partial central clearing (bull's-eye or targetoid appearance) — is the pathognomonic presentation of erythema migrans (EM). This occurs in 70–80% of early Lyme disease cases, appearing 3–30 days after tick bite (this patient at 9 days is classic). The clinical context is definitive: recent tick exposure in an endemic area (northeastern US), flu-like prodrome with fever, fatigue, headache, myalgias, arthralgias, and regional lymphadenopathy. Erythema migrans is the sine qua non of early localized Lyme disease caused by Borrelia burgdorferi sensu lato (transmitted by Ixodes scapularis in the NE/Midwest US). Diagnosis is clinical in endemic areas; serology is not needed and may be falsely negative in early disease. [Harrison's 21e, Lyme Disease; IDSA/AAN 2020 Guidelines]
Why each distractor is wrong
Secondary syphilis with annular lesions and systemic symptoms: While secondary syphilis can present with systemic symptoms and rash, the typical lesions are maculopapular and involve the palms/soles; annular lesions are not characteristic. The epidemiologic context (recent tick bite in endemic area) and the specific morphology of EM (centrifugal expansion with advancing border and central clearing) are diagnostic for Lyme, not syphilis. Serology would be negative in early Lyme but positive in secondary syphilis.
Erythema multiforme triggered by recent viral infection: Erythema multiforme presents with fixed, symmetric lesions on extremities and mucous membranes with true target lesions (three distinct zones: central blister/necrosis, pale middle ring, erythematous outer ring). The lesion in this case is a single, expanding, non-fixed annular patch without the characteristic three-zone target morphology. EM is not associated with regional lymphadenopathy or the specific flu-like prodrome of Lyme.
Granuloma annulare with superimposed cellulitis: Granuloma annulare is a benign, chronic dermatosis presenting with asymptomatic annular papules, often on the dorsal hands and feet. It does not present acutely with systemic symptoms, fever, or regional lymphadenopathy. The rapid centrifugal expansion and systemic manifestations are incompatible with granuloma annulare.
High-YieldNEET PG
Erythema migrans is the clinical diagnosis of early Lyme disease in endemic areas; it is the sine qua non and does NOT require serology — serology may be falsely negative in early disease (antibodies take 2–4 weeks to develop).
