A 32-year-old pregnant woman from southeastern Connecticut presents to her physician in July at 26 weeks' gestation because of a skin lesion. She reports she has had fatigue, arthralgia, and headache for 2 days and a rash in her left axilla for 1 day. She lives in a wooded area and works in her garden frequently. Six weeks earlier, she had removed a small tick that was attached behind her right knee. On physical examination, she is afebrile. She has an erythematous, oval macular lesion, 7 to 8 cm in diameter, in her left axilla, with enhanced central erythema; no other abnormalities are noted. How should her case be managed?