A 10-year-old boy is brought to the emergency department in June after 4 days of fever, severe headache, and diffuse muscle aches. His family recently returned from a camping trip in rural North Carolina, and his parents recall removing a tick from his leg 5 days ago. Temperature is 39.8°C. Physical examination reveals non-blanching, erythematous macules on the wrists, ankles, palms, and soles that appeared yesterday and are now spreading toward the trunk. No skin ulcers or enlarged lymph nodes are found. Laboratory studies show platelets 58,000/µL, AST 78 U/L, ALT 66 U/L, and serum sodium 128 mEq/L.
Which of the following organisms is most likely responsible for this patient's illness?