**Question:** Following is the etiological agent of Rocky Mountain spotted fever:

A. Rickettsia rickettsii
B. Escherichia coli
C. Staphylococcus aureus
D. Neisseria meningitidis

**Core Concept:**
Rocky Mountain spotted fever (RMSF) is a severe tick-borne bacterial infection caused by Rickettsia rickettsii. Rickettsia is a genus of obligate intracellular bacteria within the family Rickettsiaceae. These bacteria can cause a range of diseases in humans and animals, including RMSF.

**Why the Correct Answer is Right:**
Rickettsia rickettsii is the causative agent of RMSF. It is an obligate intracellular bacterium, meaning it must reside inside the host cell to survive. It is transmitted to humans via the bite of infected ticks, primarily the American dog tick (Dermacentor variabilis). RMSF is characterized by fever, headache, abdominal pain, and rash, which is why it is called spotted fever.

**Why Each Wrong Option is Incorrect:**

A. Rickettsia rickettsii is the correct answer, not Escherichia coli (E. coli), which is a gram-negative bacterium causing urinary tract infections, food poisoning, and other gastrointestinal disorders.
B. Rickettsia rickettsii is the correct answer, not Escherichia coli (E. coli), which is a gram-negative bacterium causing urinary tract infections, food poisoning, and other gastrointestinal disorders.
C. Staphylococcus aureus (S. aureus) is a gram-positive bacterium commonly causing skin infections, pneumonia, and endocarditis. It is not the cause of RMSF.
D. Neisseria meningitidis (N. meningitidis) is a gram-negative bacterium causing meningitis and sepsis. It is not the cause of RMSF.

**Clinical Pearl:**
Rickettsia rickettsii is a strict intracellular bacterium, meaning it can only exist inside the host cell. Infection with R. rickettsii leads to the production of a specific antibody response called the "rickettsialpaltovirus," which is a marker for RMSF diagnosis.

**Correct Answer Explanation:**
Rickettsia rickettsii is the correct answer as it is the etiological agent responsible for Rocky Mountain spotted fever. RMSF is a tick-borne disease caused by the obligate intracellular bacterium R. rickettsii. It is transmitted to humans by ticks, primarily the American dog tick, Dermacentor variabilis. Atypical pneumonia, myocarditis, and meningitis are common complications of RMSF. The disease is characterized by high fever, headache, and a maculopapular rash (rickettsialpaltovirus).