A 12-year-old boy, who had been camping in the mountains with his family, was brought to the emergency depament because of headache, rash, and the abrupt onset of a high fever. The rash began on his palms and soles but spread up to his ankles and arms. On physical examination he was found to have palpable purpura on his wrists and lower legs. The infection is likely caused by?

Infections with Rickettsia rickettsii, Treponema pallidum, and coxsackievirus all can cause a rash on the palms and soles. This patient is most likely infected with R rickettsii. The clinical course is typical for Rocky Mountain spotted fever, which is characterized by fever with abrupt onset and a rash that spreads from the extremities toward the trunk. Palpable purpura is a poor prognostic sign, as it indicates active vasculitis and leakage of blood into the skin. The rash in syphilis and coxsackievirus is typically more gradual.