Which clinical feature is not common to both Ehrlichiosis and Rocky Mountain spotted fever?

The correct choice highlights a key distinction between Ehrlichiosis and Rocky Mountain spotted fever regarding their mechanisms of infection and pathogenesis. Ehrlichiosis is caused by bacteria from the genus Ehrlichia, which infect and replicate within blood cells, specifically leukocytes, leading to a characteristic feature of parasitized white blood cells. In contrast, Rocky Mountain spotted fever is caused by Rickettsia rickettsii, which does not parasitize blood cells in the same way; instead, it infects the endothelial cells lining blood vessels. This fundamental difference in the pathogens' interaction with blood cells makes the presence of parasitized blood cells a clinical feature unique to Ehrlichiosis.

The other characteristics shared by both infections explain some similarities in their clinical presentation and treatment. Both conditions present as acute febrile illnesses, which means they typically manifest with fever that develops suddenly and is often accompanied by other generalized symptoms. Doxycycline is an effective and commonly used treatment for both types of infections, highlighting their similarity in pharmacological intervention. Additionally, both infections have the potential to cause severe disease and can be lethal if left untreated, emphasizing the importance of timely diagnosis and treatment.