**Core Concept**
Lyme disease is a zoonotic bacterial infection caused by *Borrelia burgdorferi* transmitted through the bite of an infected arthropod vector, primarily affecting the skin, joints, and nervous system.

**Why the Correct Answer is Right**
The correct answer is a tick because *Ixodes scapularis* (blacklegged tick) and *Ixodes pacificus* (western blacklegged tick) are the primary vectors of *Borrelia burgdorferi*, the causative agent of Lyme disease. The tick's saliva contains proteins that facilitate the transmission of the bacteria during feeding. The disease is more common in areas with high tick populations, such as the northeastern United States, the Pacific coast, and northern Europe.

**Why Each Wrong Option is Incorrect**
* **Option A:** Rat fleas are vectors of other zoonotic diseases, such as bubonic plague caused by *Yersinia pestis*, but not Lyme disease.
* **Option C:** Mites can transmit diseases like scrub typhus caused by *Orientia tsutsugamushi* but are not associated with Lyme disease.

**Clinical Pearl / High-Yield Fact**
Lyme disease is often characterized by a distinctive "bull's-eye" rash, known as erythema migrans, which appears at the site of the tick bite and can precede systemic symptoms.

**✓ Correct Answer: B. Tick**