A 36-year-old man presents to his primary care physician’s office complaining of fever and headache. On examination, he has leucopenia, increased liver enzymes, and inclusion bodies are seen in his monocytes. History reveals that he is outdoorsman and that he remembers removing a tick from his leg. Which of the following is the most likely diagnosis?

**Core Concept**
The patient's symptoms, including fever, headache, leucopenia, and the presence of inclusion bodies in monocytes, along with a history of tick removal, suggest a tick-borne illness. The disease is characterized by its impact on the hematopoietic system, liver, and the presence of specific inclusion bodies.

**Why the Correct Answer is Right**
The patient's condition is consistent with **Ehrlichiosis**, a tick-borne illness caused by the bacterium *Ehrlichia chaffeensis*. The inclusion bodies seen in monocytes are called **morulae**, which are aggregates of bacteria. The disease affects the hematopoietic system, leading to leucopenia, and also impacts the liver, causing increased liver enzymes. The tick vector, *Amblyomma americanum*, is typically found in the southeastern United States.

**Why Each Wrong Option is Incorrect**
* **Option A:** While *Anaplasma phagocytophilum* is also a tick-borne pathogen, it typically causes **Anaplasmosis**, which presents with fever, headache, and thrombocytopenia, but not typically with inclusion bodies in monocytes.
* **Option B:** **Lyme disease** is a tick-borne illness, but it is caused by *Borrelia burgdorferi* and typically presents with a characteristic "bull's-eye" rash, not inclusion bodies in monocytes.
* **Option D:** **Rocky Mountain spotted fever** is a tick-borne illness caused by *Rickettsia rickettsii*, but it typically presents with fever, headache, and a characteristic rash, not inclusion bodies in monocytes.

**Clinical Pearl / High-Yield Fact**
Ehrlichiosis is often underdiagnosed due to its nonspecific symptoms and the need for specialized testing to detect the morulae in monocytes. Clinical suspicion, especially in patients with a history of tick exposure, is crucial for early diagnosis and treatment.

**Correct Answer:** C.

✓ Correct Answer: A. Ehrlichiosis