**Core Concept**
Q fever is a zoonotic disease caused by the bacterium *Coxiella burnetii*, which is primarily transmitted to humans through inhalation of aerosolized particles contaminated with the bacteria. This disease is characterized by a range of clinical manifestations, from asymptomatic infection to severe pneumonia.

**Why the Correct Answer is Right**
The correct answer is **C. Acute onset of fever, headache, and pneumonia**. Q fever typically presents with acute onset of fever, headache, and pneumonia, which can be severe in some cases. The symptoms are often accompanied by fatigue, chills, and muscle aches. The pathogenesis of Q fever involves the inhalation of *C. burnetii* bacteria, which then infect the alveolar macrophages in the lungs, leading to a severe inflammatory response.

**Why Each Wrong Option is Incorrect**
**Option A:** Q fever is not primarily a disease of the gastrointestinal tract, although some cases may present with gastrointestinal symptoms. However, this is not the characteristic presentation of Q fever.
**Option B:** *C. burnetii* is not transmitted primarily through the bite of an arthropod, unlike other zoonotic diseases such as Lyme disease or Rocky Mountain spotted fever.
**Option D:** Q fever is not typically associated with a rash, which is a characteristic feature of other diseases such as typhus or scrub typhus.

**Clinical Pearl / High-Yield Fact**
It is essential for clinicians to be aware of the diagnosis of Q fever, particularly in individuals who have a history of exposure to animals, such as veterinarians or farmers. The diagnosis of Q fever can be challenging, and laboratory tests may be required to confirm the infection.

**Correct Answer: C. Acute onset of fever, headache, and pneumonia**