A 28-year-old woman from Himachal Pradesh presents with a 7-day history of fever, severe headache, myalgia, and rash on the wrists and ankles. On examination, petechial rash is noted on the flexor surfaces. She has no eschar. Platelet count is 92,000/μL, and serum creatinine is 1.8 mg/dL. Blood cultures are pending. She reports tick exposure while hiking 10 days ago. What is the most appropriate next step?