Which organism is the etiologic agent of Rocky Mountain Spotted Fever?

• A. Treponema pallidum
• B. Rickettsia rickettsii
• C. Borrelia burgdorferi
• D. Bordetella pertussis

Answer: (B)

Rocky Mountain spotted fever is caused by Rickettsia rickettsii, an obligate intracellular, gram-negative bacterium in the spotted fever group. It is transmitted to humans by ticks, most commonly Dermacentor species. The organism targets endothelial cells, causing vasculitis that leads to the abrupt fever, severe headache, and the characteristic rash that begins on the wrists and ankles and spreads inward. Early recognition and treatment with doxycycline are important for a good outcome.

The other organisms listed cause different diseases: Treponema pallidum causes syphilis, Borrelia burgdorferi causes Lyme disease, and Bordetella pertussis causes whooping cough. None of these are the etiologic agent of Rocky Mountain spotted fever.