**Core Concept**
Ehrlichia chaffeensis is an obligate intracellular bacterium that causes a tick-borne disease in humans, characterized by fever, headache, and anemia. This bacterium is transmitted through the bite of an infected tick, specifically the Lone Star tick (Amblyomma americanum).

**Why the Correct Answer is Right**
Ehrlichia chaffeensis infects mononuclear phagocytes, including neutrophils and macrophages, and causes a condition known as monocytic ehrlichiosis. The bacterium is internalized by the host cells, where it multiplies and causes cellular damage. The resulting immune response leads to clinical manifestations such as fever, headache, and anemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Anaplasma phagocytophilum is another tick-borne bacterium that infects neutrophils, but it is not caused by Ehrlichia chaffeensis.
**Option B:** Rickettsia rickettsii is the causative agent of Rocky Mountain spotted fever, transmitted by the American dog tick.
**Option C:** Bartonella henselae is a bacterium that causes cat-scratch disease, transmitted through scratches from infected cats.
**Option D:** Coxiella burnetii is the causative agent of Q fever, transmitted through inhalation of contaminated dust.

**Clinical Pearl / High-Yield Fact**
Tick-borne diseases are an important consideration in the differential diagnosis of fever, headache, and anemia, especially in patients with a history of tick exposure.

**Correct Answer: A. Ehrlichia chaffeensis**