Final answer:
Rocky Mountain Spotted Fever is caused by the bacterium Rickettsia rickettsii, which is transmitted by ticks and can lead to severe health conditions.
Step-by-step explanation:
Rocky Mountain Spotted Fever (RMSF) is a serious infectious disease caused by the bacterium Rickettsia rickettsii. This disease is transmitted through the bite of infected hard-bodied ticks, such as the American dog tick, Rocky Mountain wood tick, and brown dog tick. The correct answer to the question is a) Rickettsia rickettsii, which is an obligate intracellular parasite responsible for causing both RMSF and typhus. It's important to note that RMSF can lead to life-threatening conditions such as meningoencephalitis, an inflammation of the membranes surrounding the brain, if not properly diagnosed and treated.