An 8-year-old Cub Scout who returned from an outing 9 days ago is brought to the clinic with the rapid onset of fever, headache, muscle pain, and rash. The maculopapular rash began on the flexor surfaces of the wrist and has become petechial as it spread inward to his trunk. Which of the following is the most likely diagnosis?
a) Measles
b) Rocky Mountain spotted fever
c) Scarlet fever
d) Lyme disease